Site 900 is situated in the Iberia Abyssal Plain over a roughly circular basement high (Fig. 2 in "Site 897," this volume) within the presumed ocean/continent transition (OCT) zone off western Iberia (see "Introduction" chapter, this volume). The site is one of a transect of drill sites across the OCT designed to study the petrologic changes in the basement rocks within the OCT as a means of identifying the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. The site is in a region of weakly magnetized, presumed thinned continental crust (see Fig. 4 in "Introduction" chapter, this volume). An RCB hole was drilled with the primary scientific objective of penetrating basement to a depth sufficient to firmly establish its character. The hole was drilled and cored to 805 mbsf and passed through Pleistocene to late Paleocene age sediments and 56 m of mafic igneous rock. Coring was terminated when the rate of penetration slowed to 1 m/hr and bit failure was imminent. A total of 380 m of sonic, resistivity, and FMS logs was acquired from three separate intervals in the sediments and basement.
The first event in the history of Site 900, which we can deduce from the cores, is the formation of the mafic igneous rock that now forms the basement here. At present, we can only speculate about the origin of this rock. It may be (1) cumulate gabbro (of any pre-late Paleocene age) either formed in, or possibly underplated at the base of, continental crust, or (2) pre-Mesozoic mafic rock involved in the Hercynian orogeny and later incorporated in the passive margin. The rock subsequently experienced a series of deformation and metamorphic events. The rock has a well-marked foliation, which attests to high-temperature shear deformation. This was followed by retrograde metamorphism to low-grade amphibolite or greenschist facies. An ensuing lower-temperature deformation is expressed in narrow shear zones, fractures and veins, and brecciation. The density of the basement rocks is about 2.6 to 2.9 g/cm3; their velocity ranges from 3.7 to 7.5 km/s, with a cluster of observations at 5.7 km/s. One effect of the metamorphism seems to have been the virtual destruction of any remanent magnetization.
Preliminary comparison with basement rocks from Sites 897 and 899 suggests that this tectono-metamorphic evolution may be common to each site, although the primary lithologies are different. Finally, the metamorphosed mafic igneous rocks were exposed at the seafloor prior to the late Paleocene, probably by the Early Cretaceous rifting.
The sediments cored at this site reveal a late Paleocene to Pleistocene history of sedimentation on an evolving continental margin, starting with predominantly contour current reworking of turbidites, followed by the dominance of turbidite deposition from the middle Miocene onward. Sedimentation over this interval ranged between 13 and 27 m/m.y.
From the late Paleocene until the early Miocene, a sequence of clay or silt with nannofossil clay was deposited. Sedimentation was interrupted for about 7 m.y. during the early Eocene. An important characteristic of these sediments is the presence of upward-darkening sequences. These range in thickness from 10 to 30 cm, usually have sharp bases and tops, and are intensely bioturbated. These features and small-scale structures in the cores point to reworking by contour cur rents. At the same time, evidence of downslope sediment movement can be seen in the form of scattered mud turbidites. A series of calcite-cemented sandstones, deposited until the earliest Miocene, may represent a combination of turbidity and contour-current deposition. Above a fault of unknown displacement in the early Miocene sequence, possibly the result of synsedimentary deformation, upward-lightening sequences appear that are indicative of turbidite deposition. Pervasive bioturbation indicates deposition above the CCD.
The sequence from the early Miocene to the Pleistocene is principally nannofossil clay and ooze. The turbidites were dominated by mud, and sandy bases are uncommon. The turbidite bases are mostly siliciclastic, but occasionally carbonate-rich, suggesting two distinct provenances. Turbidite deposition ceased from late Miocene to late Pliocene. Bioturbation is pervasive. A 4.0-m.y. hiatus, beginning in the middle Miocene (around 12 Ma), correlates with a regional angular unconformity in seismic-reflection profiles; this may be related to northwest-southeast compression on this margin during a compressional phase in the Betic Mountains in southern Spain and structural inversion in the Lusitanian Basin of Portugal.
Until more work is done on the basement rocks from this site, it is not possible to recognize the extent to which they will contribute to our understanding of the ocean/continent transition on this margin. Post-cruise studies may be expected to produce new understanding of these complex rocks.