7. SITE 900¹

Shipboard Scientific Party²  

HOLE 900A

Date occupied: 10 May 1993

Date departed: 22 May 1993

Time on hole: 11 days, 20 hr, 15 min

Position: 46°40.994'N, 11°36.252'W

Bottom felt (rig floor; m, drill-pipe measurement): 5048.5

Distance between rig floor and sea level (m): 11.72

Water depth (drill-pipe measurement from sea level, m): 5036.8

Total depth (rig floor; m): 5853.50

Penetration (m): 805.00

Number of cores (including cores having no recovery): 86

Total length of cored section (m): 805.0

Total core recovered (m): 519.61

Core recovery (%): 65

Oldest sediment cored:

Depth (mbsf): 739.86
Nature: claystone
Age: late Paleocene Measured velocity (km/s): 2.40

Hard rock:

Depth (mbsf): 748.90
Nature: altered mafic igneous rocks
Measured velocity (km/s): 5.8

Principal results: Site 900 is situated in the Iberia Abyssal Plain within the ocean/continent transition (OCT) zone over an angular basement high that has some similarity to a tilted fault block. Geophysical modeling had indicated that this basement high lay within a part of the OCT having a very weak magnetization and thus was probably thinned continental crust. The site was 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. Cores were obtained from a hole that penetrated 748.9 m of Pleistocene to Paleocene sediments and 56.1 m of basement composed of retrograde metamorphosed mafic igneous rocks. Coring was terminated when the rate of penetration came close 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.

1. Basement is composed of mafic igneous rocks that underwent retrograde metamorphism to greenschist to amphibolite-grade facies. The history of these rocks is unclear at present; they may have originated as Paleozoic intrusive or metamorphic rocks, or pre-late Paleocene continental crust or subcontinental mantle that was invaded by gabbro. In any case, they were exposed at the seafloor prior to the late Paleocene, probably by Early Cretaceous rifting.
2. A 4-m.y. hiatus during middle/late Miocene time, which began at 11.7 Ma, is correlatable with a regional angular unconformity in seismic reflection profiles and may be related to northwest-southeast compression on this margin during a compressional phase in the formation of the Betic Mountains in southern Spain and structural inversion in the Lusitanian Basin of Portugal.

Two lithostratigraphic units were identified at Site 900.

1. Subunit IA (0-67.2 mbsf) is a Pleistocene to late Pliocene age nannofossil clay, clay, and nannofossil ooze. The subunit mainly consists of mud-dominated turbidites and hemipelagic/pelagic sediments.
   Subunit IB (67.2-96.0 mbsf) is a late Pliocene to late Miocene age nannofossil claystone and nannofossil chalk. The subunit consists mainly of hemipelagic/pelagic sediments.
   Subunit IC (96.0-181.5 mbsf) is a late Miocene to late early Miocene age nannofossil claystone, claystone, and nannofossil chalk with minor siltstone and fine sandstone. The subunit consists mostly of mud-dominated turbidites and hemipelagic/pelagic sediments.
2. Subunit IIA (181.5-234.3 mbsf) is an early Miocene age nannofossil claystone and chalk, with claystone and minor silty claystone to fine sandstone. The subunit consists mainly of contourites and turbidites with pelagic/hemipelagic sediments.
   Subunit IIB (234.3-748.9 mbsf) is an early Miocene to Paleocene age silty claystone to clayey siltstone, nannofossil claystone, and claystone with calcareous silty claystone to clayey siltstone, siltstone, and fine sand stone. The subunit consists mainly of sediments, possibly including turbidites reworked by contour currents.

The sediments at this site reveal the history of development of the lower continental rise adjacent to the Iberia Abyssal Plain during Cenozoic time. The cores chronicle the deposition of silt and clay layers with laminated bases under the influence of bottom currents, which were probably con tour-following currents and part of the general oceanic circulation. Mud turbidites were occasionally seen, too. These sediments were succeeded by carbonate-rich turbidites and then by mud-dominated turbidites as the abyssal plain sediments built upward and sideways onto the rise.

The cores provide a discontinuous fossil record from Pleistocene through late Paleocene time. Calcareous nannofossils generally are abundant to very abundant. Planktonic foraminifers generally are common to abundant in the upper section of the hole, but samples from the deeper sections contained fewer specimens. Two major hiatuses representing the early late Miocene and the early Eocene, as well as several minor hiatuses, have been identified.

The Matuyama, Gauss, and Gilbert chrons tentatively were identified from paleomagnetic measurements of the sediments above 145 mbsf. Below that depth, the sediments are very weakly magnetized and no chrons have been identified. The metamorphic basement rocks do not provide any reliable magnetic results, mainly because of weak magnetization. Magnetic susceptibility values generally follow the pattern of remanent magnetization values in both sediments and crystalline rocks.

Fifty-six meters of fine- to coarse-grained metamorphosed mafic rocks were drilled in the basement. The rocks are highly deformed and brecciated and veined by later calcite, epidote, and clinozoisite. A porphyroclastic texture having large porphyroclasts of plagioclase and clinopyroxene in a recycled matrix of the same minerals can be seen in thin section. Chemical analyses suggest that the rocks are relatively depleted in large ionic lithophile elements. These rocks may be (1) Paleozoic mafic rocks that were accreted onto continental basement during the Hercynianorogeny, (2) Cumulate gabbro (of any pre-late-Paleocene age) either formed in, or possibly underplated at the base of, continental crust. The rocks subsequently experienced a series of deformation and metamorphic events. In any case, they were exposed at the seafloor prior to the late Paleocene, probably by the Early Cretaceous rifting.

The sonic, resistivity, and Formation Microscanner (FMS) logging strings were run over three separate parts of the hole, including 36 m of basement. Hole conditions made logging difficult and forced us to use the conical side-entry sub. Eventually, logging had to be abandoned because of persistent obstructions in the hole and damage to the logging cable and FMS tool.

Measurements of physical properties in the sediments indicate 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. The clay-rich sediments were notable for their significant seismic anisotropy in places (more than 7%) and relatively strong vertical velocity gradient (1 s^-1), compared to that at Sites 897, 898 and 899. The density of the basement rocks is about 2.6 to 2.9 g/cm³, and their velocity ranges from 3.7 to 7.5 km/s, with a cluster of observations at 5.7 km/s.

Interstitial-water samples were obtained from lithologic Units I and II (13-722 mbsf). The principal result is a steady downward decrease in concentrations of sulfate throughout the hole, from a value near that of sea-water concentration at 13 mbsf to a minimum of 1.9 mM at 702 mbsf. The profile is slightly convex upward, suggesting that some sulfate reduction has occurred within the sedimentary column. Concentrations of ammonia are consistent with this interpretation. A similarly shaped magnesium pro file may result from clay mineral alteration. Concentrations of calcium and strontium suggest carbonate recrystallization at about 300 mbsf and below 636 mbsf.

Profiles of carbonate content vs. depth reflect a history of generally low biological productivity and deposition of hemipelagic sediments be low the carbonate compensation depth (CCD), combined with delivery by turbidites of carbonate-rich material initially deposited above the CCD. An average 0.3% organic carbon was found in Unit I; this is much less than that found at Sites 897 and 898. Variable organic C/N ratios from Unit I indicate the fluctuating predominance of marine or terrigenous sources of organic matter. Concentrations of biogenic methane encountered in head space gas analyses of lithostratigraphic Unit I to Subunit IIB generally are low, as were those found at Site 899. Methanogenesis may have been inhibited by interstitial sulfate, as indicated by the generally high sulfate concentrations in the pore waters.

¹Examples of how to reference the whole or part of this volume can be found under "Citations" in the preliminary pages of the volume.
²Shipboard Scientific Party addresses can be found under "Shipboard Scientific Party" in the preliminary pages of the volume.

Ms 149IR-107
Reproduced online: 15 October 2004