A transect of four closely spaced sites was drilled on the landward edge of the Iberia Abyssal Plain during Leg 149 (Fig. 1). Seismic surveys performed in preparation for Leg 149 identified a series of seaward-dipping reflectors on the landward edge of the Iberia Abyssal Plain that are overlain by a horizontal succession of sediment layers. The seismic profile spliced together from three east-west surveys suggests that the sediment layers are continuous across most of the drill sites (Fig. 2). Sediments obtained from the drilling transect revealed that the seismic reflectors represent a succession of Miocene-to-Pleistocene turbidite layers that vary somewhat in composition and thickness among the four sites (Table 1). Concentrations of CaCO3 are quite variable in the sediments (Fig. 3, Fig. 4, Fig. 5, Fig. 6). This variability reflects the intermix of pelagic and hemipelagic sediment types in these turbidites, which have contents ranging from nannofossil ooze to fine sand (Shipboard Scientific Party, 1994a-d).
Sediments of Pliocene-Pleistocene Unit I at Site 897 and 898 are composed of interbedded distal turbidites that accumulated rapidly. Individual turbidite sequences range in thickness from 5 cm to over 1 m (Shipboard Scientific Party, 1994a, b). The typical sequence consists of a layer of dark sand overlain by silty clays and capped by a thin layer of lightly colored nannofossil ooze. The dark color of the sands is attributed to higher organic carbon contents in these layers. The small contribution of biogenic ooze indicates that pelagic sediments constitute a minor amount of the unit. Averaged sedimentation rates for Unit I range from 90 m/m.y. at Site 898 to 532 m/m.y. at Site 897 (Shipboard Scientific Party, 1994a, b).
Drilling at Site 899 recovered sediments from the basal 50 m of a 132-m-thick Pliocene-Pleistocene layer of terrigenous distal turbidites. These turbidites are comparable in composition and texture to those at Sites 897 and 898, but they accumulated less rapidly. Sedimentation rates are estimated to have averaged 35 m/m.y. (Shipboard Scientific Party, 1994c). The Pliocene-Pleistocene sediments at Site 900 consist of siliciclastic muddy turbidites that accumulated much less rapidly than they did at Sites 897 and 898. Individual sequences range in thickness from 10 cm to more than 1 m and are separated by layers of nannofossil ooze up to 60 cm thick (Shipboard Scientific Party, 1994d). Averaged sedimentation rates range from 22 to 24 m/m.y.
Underlying Miocene sediments at all four sites were more pelagic, accumulated more slowly, and contained fewer turbidite sequences than the Pliocene-Pleistocene sequences. The averaged sedimentation rates for these sediments ranged from 2 to 13 m/m.y. (Shipboard Scientific Party, 1994a-d).
The differences in the accumulation rates of the four sites may result from differences in sediment transport routes, and bottom topography is a part of this explanation. The sites were positioned above basement highs to more easily recover basement rock. The sedimentary units above the basement highs are typically thinner than in the surrounding basins (Fig. 2) because of sediment focusing into the topographic lows. Sites 897 and 898 were drilled prior to loss of drill string, which subsequently limited drilling to the thinner sedimentary sequences present at Sites 899 and 900. The four Leg 149 sites consequently provide a unique comparison of diagenesis of sedimentary organic matter at closely spaced locations having different accumulation rates.