Site 897 (Fig. 1) 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 in the "Introduction" chapter (this volume) and elsewhere (Whitmarsh et al., 1990; Whitmarsh et al., 1993). Site 897 was chosen to sample the region where the thin oceanic crust to the west merges with the intermediate zone of high magnetization and smooth acoustic basement to the east. The site is located on the easternmost of a series of roughly north-south basement ridges (Fig. 2 and Fig. 3). The ridge under Site 897 may have the same tectonic relationship to the OCT as the peridotite ridge drilled at Site 637 during Leg 103 (Beslier et al., 1993). The latter ridge is widely thought to represent the boundary between oceanic and continental rocks off western Galicia Bank (Boillot et al., 1980). Although both sites are situated over linear ridges, which can be followed for tens of kilometers in a north-south direction, the ridges are not continuous but may be linked via two en-echelon sinistral offsets, possibly representing successive stages of south-to-north rifting along this margin (Beslier et al., 1993).
About 680 m of sediment overlies basement. By analogy with Site 398, 145 km to the east of Site 897, the lithologies were expected to be ooze/chalk with turbidites over chalk, mudstone, and claystone. From seismic reflection profiles traced back to Site 398 before Leg 149, the basal sediments were estimated to be as old as Maastrichtian. The Miocene regional unconformity, which resulted from gentle folding that occurred during the Rif-Betic Compressional phase farther south and which is visible clearly in seismic reflection profiles from the Iberia Abyssal Plain, was expected to occur at about 360 mbsf. In the vicinity of Site 897, the unconformity is marked by horizontal sedimentary reflectors that onlap low-angle west-dipping reflectors; this can be seen in the east-west seismic profile across the site (Fig. 3). The sediments thicken to 2.0 s (2.4 km) in a basin west of the site and to 1.4 s (1.5 km) to the east. Although we anticipated that the acoustic basement at Site 897 might contain ultramafic rocks within a few hundred meters of its upper surface, we were not certain that these had ever actually outcropped, so that the uppermost basement was expected to be any mixture of lithology from continental basement rocks to igneous intrusive/extrusive material to tholeiitic lavas of the upper oceanic crust.