m), as are seismic velocity (4.2-6.9 km/s), and thermal conductivity (1.4-2.5 W/m·K).
Date occupied: 29 April 1993
Date departed: 10 May 1993
Time on hole: 11 days, 14 hr, 24 min
Position: 40°46.347'N, 12°16.063'W
Bottom felt (drill-pipe measurement at rig floor, m): 5302.5
Distance between rig floor and sea level (m): 11.54
Water depth (drill-pipe measurement from sea level, m): 5291.0
Total depth (from rig floor, m): 5865.00
Penetration (m): 562.50
Number of cores (including cores having no recovery): 37
Total length of cored section (m): 332.0
Total core recovered (m): 173.98
Core recovery (%): 52
Oldest sediment cored:
Depth (mbsf): 549.70 Nature: claystone Age: Barremian Measured velocity (km/s): 4.30
Principal results: Site 899 is situated in the Iberia Abyssal Plain, within the ocean/continent transition (OCT) zone over a semi-elliptical basement ridge having a steep southern flank. Geophysical modeling had predicted that the ridge lay within a part of the OCT, intermediate between thin oceanic crust to the west and thinned continental crust to the east, in which a large positive magnetic anomaly that cannot be modeled by seafloor spreading is found. The site is one of a transect of sites across the OCT designed to study the petrologic changes in the basement rocks within the OCT to identify the processes that accompanied continental breakup and the onset of steady-state seafloor spreading. Rotary cores were obtained from two holes that penetrated a total of 562.5 m of late Pliocene to Late Cretaceous age sediments overlying an unusual serpentinite breccia and serpentinized peridotite having interbedded pre-Late Cretaceous to late Barremian claystone and siltstone. Coring was terminated when the drill string became temporarily trapped near the bottom of the hole. Downhole logs were acquired from within acoustic basement in the interval from 395 to 430 mbsf; deeper logging was prevented by bridges and, when we tried to log from the base of the casing to the top of basement, bridges stopped the logging tools in that interval, too.
Four lithologic units were identified at Site 899.
The most unusual aspect of the sequence of rocks recovered is Unit IV. It consists of three lithologies. These are (1) serpentinite breccias units, up to 95 m thick; (2) unbrecciated serpentinized peridotite and gabbro; and (3) intercalated claystone, unconsolidated altered serpentinite, and minor siltstone associated with basalt fragments as individual pieces in the cores or as clasts within the sediment. The clasts in the breccia units are up to 1 m thick. At least 90% of these clasts are serpentinized peridotite, many of which display a fabric indicative of moderate-temperature/high-pressure ductile deformation, and the remainder are fragments of metamorphosed magnesium-rich igneous rock. These breccia units are largely structureless and unsorted. Rarely, flow structures and the sediment/breccia contact are seen near the bases of the breccia units. The matrix has a texture that, in a tectonic setting, indicates shear deformation under low normal stress that typically results from high fluid pressures. Clasts of continental basement rocks and sediment are entirely absent, but clasts possibly composed of basalt and microgabbro are present. Below the breccia units, Subunit IVB is principally unbrecciated boulder-sized blocks of serpentinite, but also contains intercalated siltstones and claystones and intercalated clasts of basalt, microgabbro, and chlorite-bearing mylonite. This subunit is interpreted as a series of mass flow deposits. Both subunits contain intercalated Early Cretaceous age sediments exhibiting a normal stratigraphic sequence that gets younger uphole.
The cores at Site 899 provide a discontinuous fossil record from the late Pliocene through Late Cretaceous. Calcareous nannofossils generally are present, but sometimes are rare in the Cretaceous section. Planktonic foraminifers are abundant to common in the late Pliocene to late early Miocene age deposits, but are less common or absent below. A hiatus representing at least 2.9 m.y. lasted from middle Miocene to late Miocene time. A major hiatus representing most of the Paleogene lies within Core 149-899B-15R. The presence of the Early Cretaceous age nannoflora is especially important for providing a tentative stratigraphy for the serpentinite breccias.
Acoustic formations 1Aand 1B are widely recognized in multichannel seismic reflection profiles on the western Iberia margin and have been locally dated by previous drilling. At Site 899, acoustic formation boundary 1A/1B correlates with the middle/late Miocene hiatus at 170 mbsf.
A few potential paleomagnetic reversals were observed in the sedimentary section. Sediment natural remanent magnetization (NRM) intensities range from 0.1 to 1.0 mA/m. Within Unit IV, the serpentinite breccias have a very stable normal magnetization that may correspond to a period within the Cretaceous Long Normal Superchron. Between 470 and 520 mbsf, a possible pattern of reversals was observed. Magnetic susceptibility was moderately high in Unit I (3 × 10-4 SI units), low in Subunit IIB (about 1 × 10-4 SI units), and very high within Unit IV (about 1 × 10-2 SI units).
Physical property measurements in the sediments show a small, but steady, increase in bulk density, seismic velocity, formation factor, shear strength, and thermal conductivity, and a concomitant decrease in porosity with depth. Within the acoustic basement, the altered ultramafic rocks have higher densities, but relatively low grain densities (both about 2.5 g/cm3). Electrical resistivity is particularly high in parts of the serpentinite breccia (up to 800 m), as are seismic velocity (4.2-6.9 km/s), and thermal conductivity (1.4-2.5 W/m·K).
Downhole logs were obtained from part of the thick serpentinite breccia unit in the top of Subunit IVA (395-430 mbsf). These showed a low natural gamma-ray count (5-10 API units), low aluminum content (1-1.5 wt%), a high resistivity (mean 150 and maximum 1000 m), low porosity (0%-10%) and compressional and shear wave velocities of 3.81 to 4.85 and about 2.3 km/s, respectively. Local higher velocities, resistivities, and densities appear to correlate with blocks of serpentinized perido tite. Core index and log bulk densities agree within ± 0.05 g/cm3.
Interstitial-water samples were obtained from within Unit I through Subunit IIB (81.5-350 mbsf). The sulfate concentration is almost constant (18.5-19.9 mM) in most of the hole, with a steady decrease within the lower half of lithostratigraphic Subunit IIB to a minimum of 15.5 mM. Sulfate is depleted with respect to seawater, indicating that sulfate reduction has probably occurred in the unsampled section from 0 to 81.5 mbsf. Profiles of carbonate content vs. depth reflect a history of generally low biological productivity, and deposition of hemipelagic sediment below the CCD, combined with delivery by turbidites of carbonate-rich material that had initially been deposited above the CCD. Unit I contains 0.3% organic carbon, which is much less than that at Sites 897 and 898. Organic C/N ratios from Unit I and the upper part of Subunit IIB are variable; a few values are anomalously low. Concentrations of biogenic methane encountered in headspace gas analyses of lithostratigraphic Unit I through Subunit IIIB generally are low. Methanogenesis may have been inhibited by the generally high sulfate concentrations.
