INTEGRATION OF SEISMIC PROFILES WITH OBSERVATIONS FROM THE SITE

Two multichannel seismic-reflection profiles were obtained across the site before the cruise. Lusigal Line 12 crosses the site in an east-west direction (Fig. 1) and Sonne Line 75-21 crosses the site in a north-south direction (Fig. 3). The profiles indicate a number of sedimentary reflectors that have been recognized on a regional scale in the vicinity of the Iberia Abyssal Plain; these 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). One of these reflectors, which separates acoustic formations 1B and 2 (Groupe Galice, 1979; Fig. 1), crosses the site. The reflection profile across the site is also characterized by two unusual acoustic facies in the interval between 0.58 s two-way traveltime and the seabed. 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).

Downhole sonic logs were obtained at Site 900 over just two intervals: between 137 and 238 and from 330 to 451 mbsf. This was insufficient to compute the depths of seismic reflectors. However, the results of two sonobuoy lines shot over the Iberia Abyssal Plain (Whitmarsh, Miles, and Mauffret, 1990) were used to convert from two-way traveltime to depth (Fig. 66, "Site 897" chapter, this volume). Thus, we were able to estimate the downhole depths of the 1B/2 and basement reflectors seen in the Lusigal 12 seismic-reflection profile and the depths downhole of the two acoustic facies described above. These are summarized in Table 17.

1. The hummocky sediment wave and the inclined reflector acoustic facies both lie within the 515-m-thick Subunit IIB (see "Lithostratigraphy" section, this chapter). The sediments of Subunit IIB show signs of reworking by contour currents, as well as some mud turbidites and contourites. No marked lithological variations are evident within Subunit IIB, but the appearance of thin beds of hard calcite-cemented sandstone in most cores below 296 mbsf, although much more closely spaced than the wavelength of the seismic energy, may well be the explanation for the pattern of inclined reflectors observed between 0.38 and 0.58 s two-way traveltime. Were the top of these reflectors to correlate with the top of the sandstones, then the interval velocity would be 1.59 km/s. This may not be the full explanation, however, because the sandstone beds occur well below the depth of the base of the inclined reflectors. Other explanations may exist for these reflectors that remain to be investigated.
2. Reflector R1 (Table 17) was computed as being at 540 mbsf. No clear explanation for the R1 reflector exists in terms of a lithologic or physical change at this depth.
3. The acoustic basement reflector was computed as being at a depth of 770 mbsf. It correlates with the top of the sequence of metamorphosed mafic igneous rock encountered at 748.9 mbsf. The computed interval velocity from 0 to 748.9 mbsf is 1.87 km/s.