Vertical reflection profiles made with a large volume (7628 cm3) sound source and airgun-sonobuoy reflection and refraction profiles in the southwest Pacific Ocean basin reveal significant accumulations of acoustically reverberant material that is difficult to penetrate with small (328--656 cm3) airguns.

An isopach map of the reverberant layers, called 'smooth acoustic basement' or the 'opaque layer' in earlier studies, is presented along with an isopach map of the overlying weakly stratified (and low velocity) sediment. The map shows that misidentification of the reverberant material as igneous basement would result in average basement depth about 500 m too shallow, and in three separate areas, 1500 to 2000 m too shallow. The new data generally agree with depth versus age curves of basement in the oldest Pacific crust as determined by DSDP boreholes.

DSDP cores from the reverberant layer in the central part of the study area show that it is volcanogenic in some places and calcareous in others. These drill sites are in areas where the reverberant layer is relatively thin, so that thick accumulations to the south have not been sampled. The calcareous nature of these thick beds is implied by the lack of consistently nearby seamount clusters, which could provide volcanogenic materials. If the thick reverberant layers are calcareous, that is, deposited at or near the equator, the net northward drift during the past 130 m.y. (or more) has been only 7--8¿ of latitude.