Leg 173 Scientific Report


Site 1067

Site 1067 lies in a water depth of 5021 m near the northern edge of the Iberia Abyssal Plain. The primary objective at this site was to drill and core down through acoustic basement (supposedly early post-rift or synrift sediment or crystalline basement) to a strong extensive intrabasement reflector believed to represent a major synrift tectonic contact. This objective could not be met directly for technical and logistical reasons but was partly achieved by drilling at an offset location (Site 1068).

The principal result of this site was identifying a 92-m-thick sequence of mafic rocks (amphibolite, tonalite gneiss, and meta-anorthosite) that probably represents differentiated products of mantle melting with mid-ocean ridge basalt (MORB) tholeiitic affinities. They could represent former gabbro retrometamorphosed under amphibolite facies conditions. The meta-anorthosite and the tonalite gneiss are intrusive in the amphibolites. The intensity of the ductile deformation decreases from the top toward the bottom of the cored section. This observation suggests that a ductile shear zone developed in the amphibolite at a level which is now at the top of the basement.

Coring commenced at 648 mbsf in Hole 1067A, and 118.2 m of sediment was drilled before basement rocks were encountered at 763.8 mbsf (Fig. 4). The sediments are middle Eocene to late Paleocene in age, and consist of claystones, calcareous claystones, calcareous silty claystones, and calcareous siltstones/sandy siltstones/sandstones. The cored interval is very similar to sediments of the same age recovered 800 m to the west at Site 900. Therefore, the sediments drilled at Site 1067 are assigned to the lower part of Subunit IIB as defined at Site 900. Upward-darkening units between 3 and 25 cm thick dominate the succession. They show a basal siltstone/sandstone overlain by calcareous claystone and capped by claystone, and are interpreted as being deposited close to the carbonate compensation depth (CCD) by turbidites, with evidence of some reworking by contour currents. A two-cm-thick conglomerate horizon encountered in Paleocene sediments near the base of lithostratigraphic Subunit IIB contains 1-5 mm granules/pebbles of pale shallow water limestones and dark gray pelite clasts that are similar in composition to clasts recovered in the Upper Jurassic and Miocene sediments drilled at Site 1065. Their presence suggests that metamorphic basement and Mesozoic shallow limestones were exposed in nearby uplifted areas, probably about 15 km to the north.

Calcareous benthic foraminifers are commonly present throughout the section, whereas agglutinated foraminifers occur only sporadically. Planktonic foraminifers in the core-catcher samples vary in abundance and preservation; this is thought to be facies controlled. A few horizons of fossiliferous foraminiferal chalk were identified. The interval between 658.4 nd 763.8 mbsf is assigned a Paleogene age based on planktonic foraminiferal recovery. All Okada and Bukry calcareous nannofossil zones are accounted for from Subzone CP13c (Zone NP15 of Martini) in Core 1067A-1R to Subzone CP8 (NP9) in Core 13R (representing early middle Eocene to late Paleocene).

Basement was first recovered at a depth of 763.8 mbsf. The basement consists largely of strongly to weakly foliated amphibolite. Tonalite gneiss and weakly to moderately deformed meta anorthosite occur as veins or patches distributed throughout the recovered sections. Matrix supported to grain-supported amphibolite breccia is also present. The basement has been divided into three units reflecting the dominant structural features (Fig. 4). Basement Unit 1 consists of strongly foliated amphibolite (locally brecciated) with regions of tonalite gneiss and minor epidosite. Unit 2 consists of matrix-supported amphibolite breccia with minor amphibolite, tonalite gneiss, moderately deformed meta-anorthosite, and cataclasite. Unit 3 consists of foliated to weakly foliated amphibolite with disseminated meta-anorthositic veins and patches. Within the Unit 3 amphibolite, relict igneous textures (metagabbro) are locally preserved. The Y and Zr contents of amphibolite and tonalite gneiss from Site 1067 are shown in Figure 5.

Foliation in the basement rocks is defined by elongate amphibole crystals, plagioclase lenses, and quartz ribbons, depending on the mineralogy and the intensity of the deformation. Dynamic recrystallization during shear deformation is extensive in quartz ribbons and locally observed in plagioclase lenses. Deformation intensity appears to decrease downward. The foliation is steeply dipping in the upper part of the cored section and less steep farther down (never less than 35°). Tonalite gneiss and meta-anorthosite occur as layers parallel or oblique to the foliation. Various types of folds occur throughout the section. Later fracturing, which is extensively but unevenly developed in the rocks, resulted in veins filled mainly with epidote, chlorite, and calcite and evolved locally into brecciation, particularly in Unit 2. In places they appear to be randomly oriented, but elsewhere epidote and chlorite veins tend to be shallow and are crosscut by steep calcite veins. Small-scale normal faults with steeply plunging slickensides occur throughout the section.

The physical properties of the claystones and sandstones fall into two distinct populations. Sandstone porosities are generally less than 20% and bulk densities greater than 2.4 g/cm3, whereas claystone porosities are greater than 20% and bulk densities less than 2.4 g/cm3. Compressional wave velocities in the claystones are relatively constant (about 2400 m/s). Sandstone velocities appear to increase with depth from less than 4000 m/s at 650 mbsf to 5700 m/s at 714 mbsf. Bulk densities of basement rocks vary from ~2.1 to 2.9 g/cm3. Grain densities are uniformly high and relatively constant about a mean of 3.0 g/cm3. Porosity is generally 10% or less. Velocities appear to be lithology dependent; breccias are less than 5000 m/s and amphibolites range from around 5000 m/s to over 6000 m/s, depending, most likely, on the amount of fracturing and alteration.

Pass-through measurements of the sediments yielded dominantly positive inclinations (normal polarity) indicating weak natural remanent magnetiziation (NRM) intensities. Progressive alternating field (AF) and thermal demagnetization failed to substantiate any polarity changes. The data suggest the presence of multiple overprints that may be related to phases of magnetite reduction and iron sulfide generation. Thus, it was not possible to construct a reliable magnetostratigraphy. Medium-grained metamorphosed mafic rocks from below 763.8 mbsf showed some resistance to AF demagnetization. Preliminary results for discrete samples show both normal and reversed magnetic polarity. The thermal demagnetization data, however, indicate the magnetization is complex and renders any polarity evaluation suspect. The unblocking temperatures and coercivities indicate that the dominant magnetic mineral in these samples is magnetite.

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