Site 899 (Fig. 1 in "Site 897" chapter, this volume) was one of a series of sites drilled during Leg 149 to elucidate the nature of the top of the crust (acoustic basement) within the ocean/continent transition (OCT) beneath the Iberia Abyssal Plain. The regional background to this and the other Leg 149 sites is presented elsewhere (see "Introduction" chapter, this volume; Whitmarsh, Miles, and Mauffret, 1990; Whitmarsh, Pinheiro, et al., 1993). Site 899, an alternative site to Site 898, was chosen at sea when the loss of drill pipe prevented us from coring an adequate section of basement at Site 898. Site 899 was located 8.5 nmi (15.7 km) northwest of Site 898 and 10 nmi (18.5 km) east- southeast of Site 897 (see "Site Geophysics" section, this chapter). Site 899 was chosen to sample a basement high located between the thin oceanic crust to the west and the weakly magnetized, thinned continen tal crust to the east (see Fig. 4 in "Introduction" chapter, this volume). The site is located within an intermediate zone of high magnetization and smooth acoustic basement (between the basement highs) that is located between the latter two crustal types. The basement high under the site is roughly circular in plan view with a steep southern slope, and its shape contrasts strongly with the linear north-south basement ridges and valleys at, and west of, Site 897 (Fig. 2 in "Site 897" chapter, this volume). The basement high is not well imaged on the two JOIDES Resolution single-channel seismic profiles that cross it, but appears to have a broad rounded top with two subsidiary peaks (Fig. 1 and Fig. 2). We expected the high to be a fault block consisting of thinned conti nental crust. Although the discovery of this site was fortuitous, its loca tion even nearer than Site 898 to Site 897 was seen as advantageous for potentially allowing the OCT to be defined even more closely than it would have been had only Site 898 been drilled to basement.
A hyperbolic acoustic reflection peaking at 0.5 s two-way traveltime (about 470 mbsf) on the single-channel seismic reflection profiles across the site was conservatively chosen as acoustic basement. A shallower hyperbola peaks at 0.4 s two-way traveltime (about 370 mbsf). By analogy with Site 398, 110 km east of Site 898, we expected to encounter ooze/chalk with turbidites over chalk, mudstone, and claystone (Sibuet, Ryan, et al., 1979). Seismic reflection profiles traced back to Site 398 indicated that the basal sediments would be as old as late Eocene/early Oligocene. A regional unconformity, which resulted from gentle folding during the northwest-southeast Rif-Betic compressional phase in southern Spain and North Africa and which is visible clearly on seismic reflection profiles from the Iberia Abyssal Plain, was expected to occur at about 190 mbsf. In the vicinity of Site 899, the unconformity is marked by horizontal sedimentary reflectors that onlap low-angle west-dipping reflectors (Fig. 1). These sediments thicken to at least 1.7 s two-way traveltime (1.9 km) in the basin underlain by the smooth basement west of the site and to at least 1.5 s two-way traveltime (1.6 km) to the east. Although we anticipated that acoustic basement at Site 899 might contain continental rocks, the exact petrology of these rocks and the amount of any synrift igneous material was completely unknown.