Leg 173 Scientific Report


Site 1068

The primary objective at Site 1068 was to sample the crystalline basement just to the west of where a strong east-dipping seismic reflector, interpreted as a major synrift tectonic contact, intersects the top of the acoustic basement (i.e., to sample the lower plate to this contact). The site, located on the west flank of the same north-south trending basement high as Sites 900 and 1067 (600 m and 1400 m to the east, respectively), was chosen after technical problems rendered it impossible to reach the reflector and underlying terrane at Site 1067.

The principal result at this site was identifying ultramafic rocks that lie beneath a shallow east dipping strong intrabasement reflector overlain by metagabbros, tonalite gneisses, amphibolites, and meta-anorthosites sampled at Sites 1067 and 900. Because serpentinized peridotites were also cored directly under the sediments at Site 897, located 80 km farther west, this result indicates that mantle rocks occur, at least as isolated outcrops in basement, over a broad region in the OCT.

Two subunits are recognized in the 139-m-thick sedimentary succession cored in Hole 1068A (Fig. 6). Lithostratigraphic Subunit IIB consists of upward-darkening sequences, 3 to 15 cm thick. These consist of a lower turbiditic calcareous sandy siltstone and calcareous claystone capped by a hemipelagic claystone. In Subunit IIC, the upward-darkening sequences are up to 45 cm thick and are largely dominated by nannofossil claystones and nannofossil chalks. Occasional laminated calcareous sandstones up to 15 cm thick occur within otherwise continuous claystone intervals, and are interpreted as lag deposits because of winnowing by contour currents. The bedding is shallowly dipping (~5°).

Biostratigraphically, the calcareous claystone section of lithostratigraphic Unit II is essentially continuous and its age ranges from middle Eocene to Maastrichtian. Calcareous microfossils are common and generally well preserved at the tops of both the Eocene and Paleocene sections, but become less so toward the bottoms of those intervals, parts of which were deposited as microfossiliferous turbidites emplaced below the CCD. Planktonic foraminifers are variable in preservation and abundance. They indicate early Eocene (Zone P9) to late Paleocene (Zone P4). All Cenozoic Okada and Bukry calcareous nannofossil zones are present from Subzone CP12a (Zone NP14 of Martini) to CP1a (NP1a). An intact K/T boundary was not recovered as it has apparently been completely eroded away and/or thoroughly mixed within the bioturbated turbidites. Micula prinsii and M. murus in Section 14R-1 help to delineate the Cretaceous/Tertiary transition there.

The 42-m-thick sedimentary breccias drilled at Site 1068 are assigned to lithostratigraphic Unit IV to conform with the lithostratigraphic scheme devised during Leg 149. Subunit IVA (<14 m) consists of matrix-supported breccias, in which a few Tithonian-Lower Cretaceous limestone clasts occur, and in which the matrix consists of carbonate mudstone yielding Early Cretaceous nannofossils. It is interpreted to have been deposited by at least three separate debris flows. Subunit IVB (18.8 m) consists entirely of clast-supported breccias. Subunit IVC (8.9 m) consists of clasts showing jigsaw brecciation set in a cataclasite matrix that also contains sand- and silt sized grains of clast material. These weakly to undeformed sedimentary breccias were probably deposited as talus deposits or rock falls at the foot of a fault scarp. The breccias of Unit IV consist of angular fragments of weakly to strongly foliated amphibolites, meta-anorthosites, and meta gabbros in a calcite-rich matrix. These clasts record a retrograde metamorphic evolution and an intense deformation under conditions ranging from upper amphibolite facies to greenschist facies comparable to those of the Site 1067 amphibolites and Site 900 (Leg 149) metagabbros. Preliminary geochemical data reveal a tholeiitic compositional range that is intermediate between these two types of rocks (Fig. 5). Toward the base of Unit IV, the matrix is more chloritic and the breccias are increasingly overprinted by brittle deformation, cataclasis, and hydrothermal alteration, and rest along a tectonized contact. There is strong circumstantial evidence to suggest the hole penetrated a seaward-dipping normal fault at this depth.

Pervasively serpentinized upper mantle peridotites (60 m recovered) constitute basement Unit 1, the deepest acoustic basement unit (Fig. 6). The pervasive serpentinization of the rocks has left behind only relicts of spinels rimmed by chlorite probably derived from plagioclase, and locally from clinopyroxene, which suggests that these rocks are derived from plagioclase-bearing lherzolites comparable to those cored at Site 897 during Leg 149 (Fig. 7). Headspace gas samples from the serpentinite and serpentinite breccia contain as much as 6000 ppm methane and 1.2 ppm ethane, with no other detectable hydrocarbons. Such anomalously high methane values are probably a result of the process of serpentinization. The uppermost part of the serpentinized peridotite is a fault breccia (Subunit 1A), whereas in the lower part (Subunit 1B), a high temperature foliation is marked by elongated spinels that dip 25° to 40° toward the west-southwest according to the paleomagnetic reorientation of one interval in the cored section. This foliation is overprinted by fractures filled with serpentine and chlorite.

The physical properties of the sediments vary on a small scale according to lithology within the repetitive sequence of calcareous sandstone, calcareous claystone, and claystone. The sandstones are the most dense, least porous, and have the highest velocities. Claystones are at the opposite end of the spectrum. In general, sandstone is not as abundant as the other two lithologies and the entire sedimentary section most probably has characteristics close to that of the calcareous claystone component (velocity ~2600-2800 m/s, density ~2.3 g/cm3). Breccias in Unit IV have velocities almost double those of the overlying sediments (over 5000 m/s) and densities 10% to 20% greater (averaging around 2.6 g/cm3). Below the breccia layer there is a velocity and density decrease at the contact with the underlying serpentinite of Subunit 1A. The serpentinite has a lower density (2.3 g/cm3) by virtue both of lower grain density and higher porosity, while velocity is almost as low as in the claystones (2700 m/s).

Shipboard paleomagnetic results indicate a number of magnetic reversals between 711.3 and 855.9 mbsf. The combined biostratigraphic and paleomagnetic data indicate that these polarity zones span Chrons C29r to C21n. Wholecore magnetic susceptibility and intensity measurements and discrete samples reveal sharp peaks in brown claystone from 778.9 to 788.5 mbsf. This characteristic brown bed and the associated peaks in NRM intensity and volume susceptibility were also observed at nearby Sites 1067 and 900 and appear to provide a stratigraphic marker within the sediments of the Iberia Abyssal Plain. The bed may represent an abrupt change in the rate of supply of terrigenous material in the early Eocene. The NRM intensity and volume susceptibility of the serpentinite unit have average values consistently above 0.15 A/m and 0.002 SI units, respectively, suggesting that serpentinites could contribute significantly to magnetic anomalies observed over the OCT of the Iberia Abyssal Plain.

Because of bridges, the hole was logged in two separate runs by the Triple Combination toolstring (507-114 mbsf and 768-614 mbsf). Although only the deepest 40 m of the logged interval was cored in Hole 1068A, the interval corresponds in depth to the sedimentary section cored and partially logged at Site 900 (Leg 149), located 600 m to the east.

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