Site 898 (Fig. 1, "Site 897" chapter, "Background and Scientific Objectives" section, 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, 1993). Site 898 was chosen to sample a basement high located between the thin oceanic crust to the west and the weakly magnetized thinned continental crust to the east (see Fig. 4 in "Introduction" chapter, this volume). The site is located at the eastern edge of 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 elliptical in plan view with a steep northern slope, and its shape contrasts strongly with the linear north-south basement ridges and valleys at, and west of, Site 897 (Fig. 2, "Site 897" chapter, "Background and Scientific Objectives" section, this volume). On east-west seismic profiles, the basement high has an irregular, possibly fault-controlled, surface, whereas on a north-south profile, it has an almost flat, gently rounded surface. We expected this high to be a fault block consisting of thinned continental crust.
About 870 m of sediment overlies basement. 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). Before Leg 149, seismic reflection profiles traced back to Site 398 indicated that the basal sediments might be as old as early Late Cretaceous. A regional unconformity, which resulted from gentle folding that occurred during the Rif-Betic compressional phase in southern Spain and North Africa and which is clearly visible on seismic reflection profiles from the Iberia Abyssal Plain, was expected to occur at about 190 mbsf. In the vicinity of Site 898, the unconformity is marked by horizontal sedimentary reflectors that onlap low-angle, westward-dipping reflectors (Fig. 1). The 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 1.8 s two-way traveltime (2.1 km) to the east. Although we anticipated that acoustic basement at Site 898 might contain continental rocks, the exact petrology of these rocks and the amount of any synrift igneous material was completely unknown.