Seismic-reflection data from the two intersecting portions of JOIDES Resolution single-channel seismic-reflection profile 149-3 (Fig. 1, Fig. 2, Fig. 3) shows that the basement high on which Site 899 is located is a roughly circular feature having a diameter about 4 km at its top and 8 km at its base. It rises about 750 m above the basement in the adjacent basins. The top of the high is not clearly imaged in the unmigrated seismic profile. Two or three diffractions off the top of the high suggest that it has two or three local peaks. The depth to basement at the site is not clear. The shallowest diffraction is at about 370 mbsf, but this may be basement or may have been caused by a fault in the overlying sediment, similar to one seen in seismic-reflection data across Site 898 (Fig. 1 in "Site 898" chapter, this volume). A stronger diffraction at about 490 mbsf seems more certain to be the top of basement or to be within basement. The Miocene unconformity, perhaps as shallow as 120 mbsf at Site 899 and to the north and east of the site, deepens toward the south and west. By analogy to Site 897, where we drilled the deepest reflector that covers basement at Site 899, we expected that the oldest regionally extensive sediments would be Eocene in age. The deeper reflectors onlap the basement high at about the same depth on each side of the high.
Magnetic data (Fig. 7 in "Site 897" chapter, this volume; P.R. Miles, J. Verhoef, and R. MacNab, pers. comm., 1993) show that Site 899 is located on the northwest side of the same magnetic anomaly high on which Site 898 is located. The magnetic high is only slightly elongated in the north northwest-south southeast direction and is not aligned parallel to the margin, as are most of the magnetic anomalies farther to the west. Beslier et al. (1993) and Whitmarsh, Miles, and Mauffret (1990) interpreted seismic profiles and magnetic data, respectively, to indicate that Site 899 is located near the oceanward edge of extended continental crust.