The depositional history of the Iberia Abyssal Plain is generally characterized through its history by the influence of downslope transport of terrigenous sediments. Various types of debris-flow deposits, turbidity-current deposits, and contour-current reworking can be found. This may have been favored by low sea levels and by catastrophic events such as slope failure, triggered, for example, by earthquakes or oversteepening (Middleton and Hampton, 1976). Two sources for the sediments could be verified: (1) the input of material form the continental margin, which can be shown by the terrigenous composition of the turbidites, and (2) the input from local morphological highs, expressed by the high abundance of foraminifers. The major role of sedimentation is displayed by the narrow continental shelf incised by submarine canyons.
The sedimentology, biostratigraphy, magnetostratigraphy, and seismostratigraphy of the Iberia Abyssal Plain area suggest the following five stages for the evolution of the abyssal plain. These stages are the major features of an integrated depositional model (Fig. 17).
The last stages of rifting probably occurred between 140 and 135 Ma (Wilson et al., this volume). No evidence could be found to suggest that the basement topography established during rifting was modified by postrift deformation. For example, there is no evidence that the basement ridge at Site 897 was tectonically uplifted or rose diapirically later (however, see Whitmarsh and Sawyer, this volume). Older Cretaceous sediments (early Aptian age) consist of heterogeneous discrete sediment bodies mixed or intercalated with mafic and ultramafic rocks. Sediments in Unit IV yield an early Aptian age. The oldest sedimentary equivalent to seismostratigraphic unit 6 is of Aptian to possibly Valanginian age (Wilson et al., this volume), which increases the difficulty of interpreting the setting of the Aptian debris flows and rock-fall breccias encountered on the crest of basement highs at Sites 897 and 899.
The Eocene red clays of lithologic Subunit IIIA are interpreted as probably the product of slow accumulation of clay in an oxygenated environment, possibly below the CCD (Shipboard Scientific Party, 1994a, c, d), which agrees well with the results of other DSDP/ODP drilling programs along the northeastern Atlantic continental margin (Sibuet and Ryan, 1979; Boillot, Winterer, Meyer, et al., 1987). During the Eocene-Paleocene, the regional CCD was relatively shallow (Emery and Uchupi, 1984). At the Site 900, the most landward site, the Eocene is dominated by deposition and reworking by contour currents.
Sites 897, 898, and 900 all contain middle Miocene contourite sequences interbedded with pelagic clays. Parts of the middle Miocene to upper Pliocene are missing from Sites 897 and 898 because of a hiatus associated with crustal compression and sediment folding during the middle Miocene (Masson et al., 1994; Sawyer, Whitmarsh, Klaus, et al., 1994). The sediment sequence is more complete at Site 900 where the middle Miocene contourites pass upwards into a sequence of thin mud turbidites interbedded with pelagic sediments at around 15.8 Ma (Fig. 17). The number of turbidites gradually decreases upwards in the upper Miocene (around 9.9 to 8.2 Ma; Gervais, this volume) from Cores 149-900A-15R (124 mbsf) to 12R (98 mbsf). Between 8.4 and 5.56 Ma, turbidites disappear completely, giving way to a continuous sequence of pelagic marls.
The upper Miocene hiatus at Site 897, 898, and 899 occurs at the top of the contourite sequence and represents a sediment gap of 2.3 Ma (Fig. 17). Above the hiatus are a few meters of pelagic clays, succeeded by the Pliocene-Pleistocene turbidite sequence.
Pliocene and Pleistocene turbidites on the Iberia Abyssal Plain represent a distinct change in sediment type from the contourites, mud turbidites, and pelagic sediments beneath.
The major input of deposits from terrigenous turbidites into the Iberia Abyssal Plain starts in the late Pliocene at 2.6 Ma and follows a period of pelagic deposition that lasted at least a few million years at Sites 897 and 900. The frequency of turbidity currents varies from 20.5 flows/100 k.y. between 1.8 and 2 Ma to 64 flows/100 k.y. between 1.2 to 1.4 Ma, and reduce gradually to the present day (Wilson et al., this volume). At the eastern Site 900, the highest rate includes 8 flows/100 k.y. between 1.6 and 2 Ma. The mechanism for initiating flows appears to be earthquakes, although the onset of Pliocene/Pleistocene turbidite deposition may have been influenced by the onset of Northern Hemisphere glaciation. The coincidence of the 2.6 Ma age of onset of turbidite deposition with the onset of Northern Hemisphere glaciation (Shackleton et al., 1984) is striking. Weaver and Kuijpers (1983) showed a link between climate/sea level change and turbidite deposition on the Madeira Abyssal Plain, and Weaver et al. (1986) postulated that the Madeira Abyssal Plain would receive turbidites during periods of climatic oscillation such as the 2.6 Ma to Holocene interval. The onset of turbidite deposition at 2.6 Ma suggests that sedimentation in the Iberian Basin is at least partially controlled by climatic change. However, the frequency of turbidite input is much greater than in the Madeira Abyssal Plain and individual turbidites are clearly not related to particular sea-level changes, as they are for the Madeira Abyssal Plain. It may be that the lowering of sea level, associated with the buildup of Northern Hemisphere ice, caused a narrowing of the continental shelves, and thus allowed more sediment to be dumped beyond the shelf edge, in turn causing the margins to become unstable. Higher rates of accumulation on the upper parts of the slope may have led to unstable sediment accumulation, which could have resulted in earthquake-triggered sediment removal by turbidity flows.
A short sedimentary hiatus in the early Pleistocene, or at least a decrease in accumulation rate, was recognized at three sites (Sites 897, 898, and 900; Zhao et al., this volume). This hiatus may represent an oceanographic event that affected a significant part of the Iberia Abyssal Plain region and may also be correlated to the onset of Northern Hemisphere glaciation (Wilson et al., this volume).