Site 899 is situated in the Iberia Abyssal Plain over a basement high within the presumed 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 between the thin oceanic crust to the west and the weakly magnetized, thinned continental crust to the east (see Fig. 4 in "Introduction" chapter, this volume), which is characterized by relatively high magnetization and a smooth acoustic basement between basement highs. Site 899 was chosen to sample an unusually shallow basement high that was discovered by chance while we were doing seismic profiling during Leg 149 and was chosen as an alternative drill site for Site 898 after the loss of about 3340 m of drill string. This site was an even more suitable site than Site 898 because it is closer to Site 897, thereby possibly allowing for stronger constraint on the location of the oceanward edge of the continent. The shallowness of the basement high also made it more accessible to drilling. The high is roughly circular in plan view with a steep southern slope and its shape contrasts strongly with that of the linear peridotite ridge at Site 897 and the north-south basement ridges and valleys further to the west (Fig. 2 in "Site 897" chapter, this volume). The high is located on the same subcircular positive magnetic anomaly as Site 898; the origin of this anomaly has yet to established. Two RCB holes were drilled at 40°46.3'N, 12°16.1'W with the primary scientific objective of penetrating basement to a depth sufficient to firmly establish its character. The first hole (899A) was washed to 81.5 mbsf and then drilled and cored from 81.5 to 235.5 mbsf to discover the depth to which casing should be set; a reentry cone and casing to 216 mbsf were installed at Hole 899B. The latter hole was drilled and cored to 562.5 mbsf until conditions at the bottom of the hole threatened the drill string and coring was stopped. Holes 899A and 899B yielded cores of Pliocene to Early Cretaceous age.
The first event in the history of Site 899, which we can deduce from the cores, was the deposition in the Barremian to Aptian, a period of about 11 m.y., of blocks of peridotite with intercalated sediments and clasts of basalt, microgabbro and chlorite-bearing mylonite. Recovery of the cores from the deepest 70 m of Hole 899B was only about 10%. The mechanism of deposition is unknown. However, because the deposit is in many ways similar to the mass flow deposit cored at Site 897, we interpret it to be a series of mass flow deposits, too. There is an unusual variety of rock clasts in the cores. The results of Sites 897 and 899, only 10 nmi apart, suggest that there could have been an extensive area of peridotite exposed at the sea bed in Barremian time, perhaps even extending continuously between the two sites. We are confident that the peridotites were brought up to the sea bed by the final stretching and break-up of continental crust as Newfoundland separated from Iberia about 130 Ma (Whitmarsh, Miles, and Mauffret, 1990). The sources of the basalt and microgabbro are uncertain. The basalt is not a MORB-type basalt, which means it is not likely to have clear oceanic affinities. On the other hand, neither microgabbro nor basalt have been reported in dredge hauls from the west Iberia margin (Capdevila and Mougenot, 1988); thus, the sources of these rocks remain enigmatic. A curious feature of these mass flows is that they were cored close to the top of the highest known basement high in the area. Because mass flows move downhill, it is hard to see how clasts from tens of miles away, or farther, reached their present position as this implies that they were transported uphill. One is forced to conclude that there may have been a different arrangement of basement relief in the Early Cretaceous and the site (which is now a high) underwent significant uplift relative to its surroundings after deposition of the mass flow. A similar hypothesis was considered for the origin of the mass flow deposit cored at Site 897.
The mass flows were followed by deposition, which we can constrain to have occurred only in late Aptian to Latest Cretaceous (Campanian/Maastrichtian) time, of at least 3 layers of serpentinized peridotite breccia. Based on dates obtained from nannofossils, the mass flows and breccia units contain intercalated sediments which young upwards. Therefore, the whole sequence appears to be a normal rock succession. Because the breccia units (1) lack clasts of continental crystalline basement rock and sediment and contain only a few possible clasts of basalt and gabbro, (2) contain mainly cataclastic textures and very angular microscopic clasts suggestive of a very short transport path, and (3) are chemically indistinguishable in bulk and trace element composition from the peridotites of Hole 897D, we conclude that their source was much more local than that of the underlying mass flows and did not allow incorporation of other rock types. The three breccia units we observed appear to have the characteristics of rapidly deposited flows with shear deformation under low normal stress typical of high fluid pressures. The unusual character of these breccias and their almost unique occurrence in the oceans makes it difficult at present to establish the mechanism of their deposition. The sole possible analog known to the Shipboard Scientific Party is the active flows drilled in the Izu-Bonin-Mariana region during ODP Leg 125 (Fryer, Pearce, Stokking, et al., 1990). These flows, however, are located in a forearc regime that at first glance appears very different from the rifted margin drilled during Leg 149. However, one can point out some clear similarities with breccias called "ophicalcites," encountered in some ophiolitic massifs of the western Alps (Queyras, Davos, Appenines). Because the Site 899 breccia units appear to be a kind of flow and were drilled near the top of the highest known basement in the area, we can reasonably postulate that they came from the same high (the fact that the "high" may have been lower during the deposition of the mass flow deposit does not rule out this conclusion as the "high" may have been uplifted between then and the deposition of the breccias). The breccia units appear to have been emplaced sometime between 18 and 65 m.y. after the estimated time of onset of seafloor spreading (Whitmarsh, Miles, and Mauffret, 1990) at this segment of the margin.
The top of the igneous/metamorphic crust probably was not sampled at this site. It is not clear if we have found unequivocal evidence at Site 899 for either a continental or an oceanic nature of the basement. In fact, it seems most likely that there is no crust at all beneath the site, only upper mantle peridotite and that the clasts transported to the site came from a geological environment characterized by a range of rock types, which have oceanic (the gabbros and radiolarian chert), non-oceanic (non-MORB basalts), and continental (felspathic siltstone) affinities. Such a region can probably be properly described as transitional between continental and oceanic crust.
After the deposition of the last breccia unit, a hiatus is seen in the depositional record until late Eocene (ca. 39 Ma). Immediately preceding this hiatus, a poorly cemented Campanian/Maastrichtian conglomerate with clasts of metasediments, claystone, and basalt was deposited. This may have developed as a weathering deposit with reworked material from the underlying breccia unit. The conglomerate is overlain by a few meters of fine-grained hemipelagic/pelagic deposits and high density turbidite or mass flow deposits, followed by carbonate-poor claystones, which resulted from slow accumulation in an oxygenated environment probably below the CCD.
The remaining sediments cored at this site reveal a history of sedimentation on an evolving continental margin, starting with current reworking of turbiditic sediments, followed by turbidite deposition. From the late Eocene until the middle Miocene, a sequence of more or less carbonate-rich claystone or siltstone having significant amounts of biogenic siliceous material was deposited above the CCD. 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 currents in an abyssal plain setting. At the same time, evidence was seen of downslope sediment movement in the form of occasional mud turbidites.
The upward-darkening sequences ended in the middle Miocene. They were succeeded by clays, calcareous clays, and nannofossil clays. For the most part, these were the result of pelagic sedimentation, but occasional turbidites also carried siliciclastic sand and silts or nannofossil clay material. Moderate to intense bioturbation indicates relatively slow sediment accumulation (computed to be from 9 to 29 m/m.y.). Deposition was in an abyssal plain setting dominated by pelagic and hemipelagic sedimentation. A 2.9 m.y. hiatus beginning in the middle Miocene (12 Ma), correlates with a regional angular unconformity on seismic reflection profiles, probably caused by northwest- southeast compressional phase in the Betic Mountains in southeastern Spain, and structural inversion in the Lusitanian Basin of Portugal. Sedimentation then continued until the end of the Miocene.
The final phase of sedimentation at this site began in the early Pliocene with the onset of turbidite deposition with interbedded pelagic/hemipelagic sediments. The overall rate of sediment accumulation increased to about 35 m/m.y. The turbidites range in thickness from 5 to 150 cm and have basal sandy silt to silty sand layers that appear to be thicker in the more recent turbidites. Bioturbation is common, but lamination is seen in only a few sands. Because we did not core the section from the seabed to 81.5 mbsf, our history of the turbidite sedimentation ends in the late Pliocene.