The final processed sections display much more information than was expected onboard the ship for this first trial in deep waters. With respect to conventional MCS data (Fig. 3, Fig. 4), the deformed sedimentary sequence as well as an internal reverse fault are more clearly imaged (Fig. 9A, B). Note that the upper 200 m of the serpentinized ridge displays acoustic reflectors (Fig. 10). These reflectors could be explained by seawater circulation and subsequent variations in the degree of serpentinization, as well as by the presence of breccias and debris-flow deposits at Sites 897 and 899, composed of peridotite and sediments (Shipboard Scientific Party, 1994). One of the main problems is always to correlate site observations with seismic data. As no downhole velocity logs were obtained at Site 897, it is not possible to directly correlate reflectors seen on seismic reflection profiles and acoustic impedance and reflection coefficients computed from physical measurements (Shipboard Scientific Party, 1994).
Based on onlap identifications (Fig. 9B), four seismic unconformities have been recognized on the Pasisar profile at depths of 170, 310, 350, and 525 m at the Site 897 location (Table 1). The acoustic unconformities at 170, 310, and 525 m are of minor importance but the one at 350 m corresponds to the disappearance of a significant portion of sediments at the base of a sedimentary section which seems to be rather complete at shotpoint (SP) 1100 (Fig. 9B). At the western end of the seismic section, unconformities at 310 and 350 m merge to become nearly single.
Holes 897C and 897D are 100 m apart and offset by less than 500 m from the Fluigal seismic line. Table 1 shows the correlation between the identified seismic unconformities and the biostratigraphic gaps, except for the deeper unconformity that corresponds to the contact of sediments with the ridge. This correlation is the best for a mean velocity of 1820 m/s. From the velocity-depth profile at the DSDP Site 398, the velocity is 1820 m/s in the upper 400 m (Shipboard Scientific Party, 1979) and 1850 and 1860 m/s from two sonobuoy experiments conducted in the vicinity of Site 897 (Whitmarsh et al., 1990).
The second important point is that the lower Miocene deformation front associated with the Betic tectonics is located at SP 390 and is clearly displayed on the Pasisar high-resolution seismic profile (Fig. 6, Fig. 9). Seismic reflectors are continuous from the undeformed to the deformed areas. The top of the deformed zone clearly merges with the lower Miocene discontinuity and the upper Eocene-lower Oligocene unconformity can be continuously followed through the deformed zone (Fig. 6, Fig. 9). Below this unconformity, the significant change in the slope of sedimentary layers at SP 900-1000 is probably related to the Pyrenean compressive phase with the presence of a fault merging with the western PR flank. A clearly defined synsedimentary reverse fault with less than a few meters vertical offset appears at SP 300. Furthermore, as the deformation front is parallel to this major reverse fault, it could correspond to the initiation of a second reverse fault with an almost zero vertical offset, which merges at the top of the PR.