INTEGRATION OF SEISMIC PROFILES WITH OBSERVATIONS FROM THE SITE

Two multichannel seismic-reflection profiles were obtained across the site before the Leg 149 cruise began (Fig. 1 in "Background and Scientific Objectives" section, this chapter; Fig. 3, in "Site Geophysics" section, this chapter). These profiles indicated a number of reflectors that have been recognized on a regional scale in the vicinity of the Iberia Abyssal Plain and that have been dated by tracing them back to Leg 103 sites west of Galicia Bank (Mauffret and Montadert, 1988) and to Site 398 near Vigo Seamount (Groupe Galice, 1979). Lusigal Line 12 crosses the site in an east-west direction and best shows the characteristics of these reflectors and the intervening acoustic formations 1A, 1B, and 2 (Groupe Galice, 1979). The upper reflector corresponds to the 1A/1B boundary and marks a regional unconformity produced by the Miocene folding that accompanied the Rif-Betic compressional episode to the south. The middle reflector corresponds to the formation 1B/2 boundary (approximately Eocene/Oligocene) and the lowest reflector to the formation 2/3 boundary (approximately Albian/Santonian). The acoustic basement is clearly seen at the base of the sedimentary section. Two additional reflectors, which occur locally around the site within acoustic formation 1B, mark the top and bottom of a sequence of prograding reflectors (see "Site Geophysics" section, this chapter).

No downhole seismic velocity measurements were obtained at Site 898, nor did coring continue deeper than 342 mbsf. Therefore, it is not possible to correlate directly between the time of reflectors seen in the seismic-reflection profiles and the various observations in cores that are referred to depth downhole. However, the results of two sonobuoy lines shot over the Iberia Abyssal Plain (Whitmarsh, Miles, and Mauffret, 1990) could be used to convert from two-way traveltime to depth (Fig. 66, "Site 897" chapter, "Integration of Seismic Profiles with Observations from the Site" section, this volume). Thus, we estimated the downhole depths of the reflectors seen in the Lusigal Line 12 seismic-reflection profile.

Part of the Lusigal Line 12 seismic section across Site 898 is presented in Figure 30, at an enlarged scale. The 1A/1B boundary is the only regional reflector that can be correlated with observations in cores. Reflector times were picked at the onset of the relevant positive pulse. One should remember that (1) the vertical resolution of the seismic profiles is approximately equal to a quarter wavelength of the predominant energy (i.e., about 15 m) and (2) the computation of reflector depth from two-way traveltime is not more accurate than 10 m.

The causes of the reflectors at the base of the turbidite sequence and at the top of the inclined reflector sequence were investigated using their computed depths and lithologic observations of and physical measurements in cores. The times and depths of all main reflectors at this site are summarized in Table 13.

1. Many reflectors can be seen in the uppermost 200 m of the section (e.g., R1) from which cores containing sand, silty sand, and silt were obtained. These lithologies are associated with the bases of numerous turbidites, and it is likely that the contrasts in acoustic impedance at the bases of the turbidites make a substantial contribution to the reflection of sound. The reflectors are more likely to correspond to the net acoustic interference pattern produced by the series of turbidites than to individual turbidites, which are much thinner than a seismic wavelength.
2. Reflector R2 (associated with the acoustic formation 1A/1B unconformity and computed to be at 200 mbsf) can be recognized in the vicinity of the site principally from the angular relationship between the tilted and folded underlying, and horizontal onlapping, reflectors. Reflector R2 may correlate with the onset at this site of a middle Miocene to late Pliocene/earliest Pleistocene hiatus that occurs somewhere between 158 and 164 mbsf. Measurements of physical properties only tentatively suggest a physical cause for this acoustic event; velocity appears to show a small stepwise increase of about 0.1 km/s at about 165 mbsf. The lithology shows a change from terrigenous turbidites above 163 mbsf to calcareous contourites and terrigenous turbidites below 172 mbsf, which correlates only poorly with the computed depth of the reflector. Nevertheless, all the above indicators are consistent for placing the reflector between 158 and 172 mbsf; this correlation was adopted here and implies an interval velocity of about 1.64 km/s from 0 to 172 mbsf.
3. Reflector R3 was computed as being at a depth of 320 mbsf. This reflector is evident in reflection profiles because of the change in the attitude of reflectors below it (i.e., they have an inclined west-or southwest-dipping sigmoidal aspect). Measurements of physical properties do not suggest a physical cause for this acoustic event, nor do the cores indicate any significant change in lithology to explain the unusual nature of the reflectors. It seems probable that when coring stopped at 342 mbsf, the bit must have passed the top of the prograding reflector sequence (the implied interval velocity for the R2/R3 interval is already high should R3 be located at 342 mbsf). However, the hole may have intersected an intervening, less reflective bed and may not have been deep enough to intersect one of the dipping beds, giving rise to the inclined reflectors, even though it had entered the inclined reflector formation itself.