Leg 173 Scientific Report


Site 1069

This site is situated in a water depth of 5150 m. It lies on a north-south seismic line over the crest of a relatively flat-topped north-south basement ridge overlain by a wedge of sediments on its east flank. Seismic data suggest that the basement ridge is a possible fault block tilted toward the continent. The site lies in a location where a variety of structural and geophysical models for the development of the OCT can be tested.

The principal conclusion at this site is that the balance of evidence indicates that the north-south elongated fault block on which it was drilled is composed of continental crust. Biostratigraphic data from Site 1069 indicate that this site was at shelf depths in the earliest Cretaceous and then subsided and had reached abyssal depths by the late Campanian. This evidence, together with very low-grade metasediments (probably occurring as clasts in a friable mixture) encountered in acoustic basement at the bottom of the hole, lead us to the tentative conclusion that the site was drilled on a continental fault block. There is no sign of the mafic or ultramafic rocks encountered at Sites 897, 899, 900, 1067, or 1068.

The middle Eocene to upper Campanian 147-m-thick sedimentary succession cored at Hole 1069A (Fig. 8) is dominated by upward-darkening sequences (2- to 55-cm thick) of calcareous siltstone or sandstone, overlain by calcareous claystone or nannofossil chalk, and capped by dark hemipelagic claystone. Two subunits are recognized in this interval based on an increase in the frequency, proportion, and thickness (up to 50 cm) of sandstone lithologies at the base of individual turbiditic sequences from lithostratigraphic Subunit IIB to Subunit IIC. These sequences are interpreted as being deposited by turbidites near or below the CCD. A 2.4-m-thick upper Berriasian to lower Valanginian yellow nannofossil chalk is defined as Unit IV. The chalk is interpreted to have been deposited as a thin pelagic drape over the underlying basement ridge above the CCD. The chalk in turn is separated from an underlying Tithonian black silt by an erosional unconformity. A 9-cm interval of Upper Jurassic (Tithonian?) hemipelagic/pelagic gray clay occurs above eight curated pieces of limestone conglomerate containing granules of pelites and meta-basic arenite. The limestone clasts consist of grainstones and algal boundstones. The thin clay and limestone pieces are defined as Subunit VA. The lithology of the limestone clasts is similar to the Upper Jurassic limestones drilled on the Galicia margin and to clasts occurring within the conglomerates in Unit V at Site 1065. The limestone clasts were probably transported and resedimented from a carbonate shelf situated to the north, probably above the present day Vasco da Gama Seamount and associated basement highs.

The calcareous nannofossil sequence at Site 1069 began in the middle Eocene (CP12b) and then passed downhole through a more-or-less continuous succession of zones and subzones to a barren interval within the Paleocene in which zones CP5 to CP4 are not represented. Interbedded hemipelagites are barren of coccoliths. The Cretaceous/Tertiary transition lies within a bioturbated turbidite sequence in Core 12R where the boundary has been obliterated. The lithologic succession then continues from the uppermost Maastrichtian to the uppermost Campanian. It is interrupted in Section 1069A-16R-2 by an unconformity, below which lies the yellowish chalk described above (Unit IV). The upper portion of the chalk is dominated by nannoconids.

Paleogene planktonic foraminifers are common and moderately well preserved in the upper part of Subunit IIB (718.8-757.4 mbsf), but, with one exception, are rare to absent and generally poorly preserved in the lower section (757.4-873.3 mbsf). Calcareous benthic foraminifers and agglutinated foraminifers are present in most core-catcher samples above Core 14R. Within the latter group, the persistent occurrence of species such as Ammodiscus spp., Bathysiphon spp., and Glomospira spp. suggests that these Paleogene sediments were deposited in a fully marine environment, probably at abyssal depths.

Pieces of very hard rock were first encountered in Core 17R (beneath the Tithonian clay and limestone clasts) and continued to the bottom of the hole (Core 25R; see (Fig. 8). These rocks (Subunit VB) consist of meta-arkosic wacke, metasiltstone, and dolomitic meta-arkose. Some of the pieces are moderately rounded and lack drilling percussion marks, which suggests that they are clasts whose original surfaces are preserved. The rocks are weakly deformed, with a metamorphic foliation defined by aligned sericite and chlorite. Relict sedimentary textures such as detrital grains and bedding/lamination are also preserved. In the siltstones, many quartz fragments are elongate parallel to foliation. Pyrite-quartz vugs are common and, in some pyrite-rich samples, thin pyritic veins and elongate pyrite-quartz vugs also parallel foliation. Core recovery in Subunit VB was extremely low, yet the rate of penetration was relatively fast (35-80 minutes per core). The only material recovered from Subunit VB (85.6 m thick) that could have been the original intraclast matrix was small amounts of dark sandy clayey silt in Cores 21R and 24R. Headspace gas from the Core 21R silt contained 101.7 ppm methane and 1.6 ppm ethane.

Metasedimentary rocks in Subunit VB contain abundant detrital quartz and plagioclase and a metamorphic mineral assemblage that includes sericite, chlorite, and in some cases dolomite. Unusually large ilmenite and leucoxene pseudomorphs (after ilmenite) appear to be metamorphic porphyroblasts, although a detrital origin in some rocks cannot be ruled out, especially for fine grained ilmenite and leucoxene. Dolomite forms subhedral porphyroblasts in one sample and may be part of the diagenetic assemblage in the second. The stable coexistence of dolomite+chlorite and dolomite+muscovite indicates very low-grade metamorphism, probably subgreenschist. Similar rocks of Paleozoic age have been reported from Galicia Bank, where they also underlie Tithonian sediment.

Overall, the bedding in lithostratigraphic Subunits IIB and C is shallowly inclined. Within Subunit IIB, a marked change occurs at 760 mbsf. Above, the true dip is ~12°, below, only ~4°. At the base of Unit II and in Unit IV the bedding is again more steeply inclined (~14°). Whereas Unit II is essentially undeformed (only some minor normal faults), Unit IV was pervasively affected by pre-Unit II soft-sediment folding and normal faulting. The deformation of the basement rocks (Subunit VB) was dominated by pressure solution. The dip of the bedding is variable in the few basement pieces that could not have rolled over in the core liner. The foliation is often subhorizontal.

The magnetic intensities of sediments in Subunits IIB and IIC are generally weak. Characteristic peaks in NRM intensity and volume susceptibility associated with an early Eocene brown bed, which was observed at four other Iberia Abyssal Plain sites, were detected in Core 7R at ~777 mbsf. Cryogenic magnetometer measurements on whole cores and on discrete samples indicate that several polarity reversals are recorded in Cores 3R through 14R (738.1-854.0 mbsf). In the lower part of Subunit IIC, below 854.0 mbsf, the sediments are entirely normally magnetized, which is consistent with the Late Cretaceous long normal superchron and the late Campanian age of the sediments.

Claystones have bulk densities from 1.94 to 2.21 g/cm3 and porosities from 31% to 44%, and calcareous claystones have bulk densities from 2.18 to 2.36 g/cm3 and porosities from 22% to 38%. Whereas sandstones have bulk densities from 2.16 to 2.34 g/cm3 and porosities from 21% to 32% in Subunit IIB, the values of bulk density and porosity are more variable in Subunit IIC (bulk density ~2.2-2.63 g/cm3, porosity ~5-30%). Compressional wave velocities have a reasonable correlation with porosity and bulk density. Metasediment samples of Subunit VB have high densities from 2.70 to 2.73 g/cm3, and high velocities from 4521 to 5639 m/s and very low porosities (<3% ). Consequently, acoustic basement at this site is identified at the top of Subunit VB.

The Triple Combination tool was run successfully between 102 and 764 mbsf. The logs overlap the cored interval between 719 to 764 mbsf (Cores 1R to 5R), which represents part of Subunit IIB, a calcareous claystone. Preliminary analysis of the logs shows a good correlation with Site 1068 between 100 and 380 mbsf and 600 and 764 mbsf. Variations of inferred sedimentation rates between sites suggest a greater rate during the Miocene at Site 1069 and a slightly lesser rate during the Eocene, relative to Site 1068. A water overpressure encountered at 650 mbsf during the final pipe trip to the rig floor may be tentatively related to a 20-m-thick interval (670-690 mbsf) where high porosity and low resistivity were recorded.

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