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During its evolution, the continental slope is frequently scoured by erosional events which may not always show up as unconformities on the seismic record. Pitfalls await the seismic interpreter, especially in the lower slope region where many of these unconformable reflectors merge and meet with the seemingly simple, conformably bedded horizons of the upper rise. In this setting, the acoustostratigraphy should be checked by drilling and refined by logging. Examples of preand post-Site 397 interpretation of profiles across the Cape Bojador continental margin are presented. They prove that slumps not clearly resolved with low-frequency airgun records mimic the continuation of reflectors from the lower slope to the rise and cause erroneous acoustostratigraphic identifications. Information about the undrilled section and the regional setting is discussed together with refraction seismic data. This results in a warning not to stratigraphically correlate layers with the same refraction velocities in the zone of facies change on the continental slope and upper rise. PRESITE REFLECTION SEISMIC INTERPRETATION Two distinct unconformities (Dt and D2 of Hinz et al., 1974) divide the sediment section on the continuous seismic records of the northwest African continental margin southeast of the Canary Islands. Before Site 397 was drilled, reflection seismic profiles gave inconclusive evidence as to whether Dt and D2 could be traced down to the continental rise over the enigmatic lower slope anticline structure. On some profiles (Meteor 25-A2 and 39-14, Valdivia 10-6 and 12) in Seibold and Hinz (1976), it seemed possible to correlate D2 with the first strong, although concordant, reflector at 4.8 sec within the uppermost rise sediments (Figure 1). On other profiles {Meteor 25-1 and 3, Valdivia 10-1, 3, 4, 8, 10 in Seibold and Hinz, 1976; Atlantis 11-117 and 118 in Uchupi et al., 1976), this "rise-D2" and the overlying Neogene reflectors appear to lap onto an escarpment. This scarp was carved into the Lower Cretaceous slope sediments by the erosional events, which are represented by Reflectors D! and D2 on the slope. The tracing of the discordancies was severely hampered by the large vertical exaggeration (> × 10) of the analog profiles. Because of many diffractions under the slope, the GSI digital profile AIL (see Site Report, this volume) could not solve this controversy. But the profile showed more than 2 km of nearly horizontal strata under the "slope anticline." This disproved the presence of a deep-seated structure such as a basement high, of which the anticline was supposed to be the surficial expression (Lancelot, Seibold, et al., 1977). Before drilling Site 397, it still seemed possible to trace the unconformities, at least on reference profile Meteor 25-2 (Figure 1), down the continental slope into the conformable marker reflectors within the upper continental rise sediments. Therefore, the sediments bounded by the reflectors under the uppermost rise were thought to represent the same time-stratigraphic unit as those found under the upper slope and shelf. The sequence there between D! and D2 was interpreted as representing transgressive Upper Cretaceous marine sediments. Hinz et al. (1974, their fig. 2) and later Seibold and Hinz (1974) thought that the post-Dj strata were transgressive, because sediments deposited immediately above Di show a concordant to onlapping relationship to the unconformity. However, the complete sequence between Di and D2 under the middle and lower slope would best be described as being constituted of of flapping sediments (Hinz, this volume). With the use of sonic log information, the underlying unconformity Dλ could be extrapolated to conformable Middle Cretaceous strata in onshore oil wells (A.U.X.I.N.I., 1969). After Mitchum and Vail (1978), the ages of unconformities are determined from well control, where the hiatuses are at a minimum (see frontispiece). Thus, Di was assumed to be of Cenomanian age and D2 was estimated to be of Oligocene to early Miocene age. LEG 41 AND 47A DRILLING INFORMATION CORRELATED WITH A NEW PROFILE (Meteor 46-37) DSDP/IPOD Sites 369 and 397 translated the geophysical parameters into geological reality. Unfortunately, however, both holes were not logged. Because

Cape Bojador Slope, an Example for Potential Pitfalls in Seismic Interpretation without the Information of Outer Margin Drilling