Site 900 (Fig. 1, "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 for this and the other Leg 149 sites is presented elsewhere (see "Introduction" chapter, this volume; Whitmarsh et al., 1990,1993). Site 900 is located about 23 nmi (43 km) east of Site 898, about 25 nmi (46 km) southwest of Vasco da Gama Seamount and 45 nmi (83 km) west-southwest of Site 398 (see Fig. 1 in "Introduction" chapter, this volume, and "Site Geophysics" section, this chapter). Site 900 was chosen to sample a basement high located near the western edge of a region of weakly magnetized, thinned continental crust that appears to extend from the OCT to the continental slope (see Fig. 4 in "Introduction" chapter, this volume). The basement high under the site is angular and only slightly asymmetric in profile, but appears as an isolated almost circular feature in plan view (Fig. 2 in "Site 897" chapter, this volume). We had interpreted the high as a fault block of continental crust. Should the acoustic basement under the site prove to be continental, then the almost 200-km-wide region of crust between the site and the base of the Portuguese continental slope might logically also be assigned a continental origin. Further, assuming symmetrical rifting, one could argue that a wide area of thinned continental crust might also exist under the conjugate Newfoundland margin, where conflicting views exist regarding the extent of thinned continental crust (Keen and de Voogd, 1988; Tucholke et al., 1989).
By analogy with Site 398, 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 Paleocene. The regional unconformity, which resulted from gentle folding that occurred during the Miocene northwest-southeast Rif-Betic compressional phase in southern Spain and North Africa, is not clearly seen in reflection profiles across the site. This may be because it is obscured by two unusual acoustic facies; the interval from 0 to 0.38 s two-way traveltime consists of hummocky sediment waves and the interval from 0.38 to 0.58 s two-way traveltime consists of a series of inclined reflectors (Fig. 1). The sediments thicken to about 1.7 s two-way traveltime (1.9 km) in the basin west of the site and to at least 2.3 s two-way traveltime (3.0 km) to the east. Although we anticipated that acoustic basement at Site 900 might contain continental rocks, the exact petrology of these rocks and the amount of any pre-rift sediments were completely unknown. Phyllites and meta-arkoses of unknown age have been dredged about 20 nmi (37 km) north of the site (Dredge C56-09; Capdevila and Mougenot, 1988).