Leg 173 Scientific Report


Site 1070

Site 1070 lies over an elongated basement ridge about 15 km east of the crest of the J magnetic anomaly and 20 km west of the peridotite ridge. Both ridges lie parallel to the continental margin. The main objective at this site was to sample oceanic basement to characterize the chemistry and melting of the early formed oceanic crust.

The principal result from Hole 1070A is that the basement consists of pegmatitic gabbro overlying serpentinized peridotite intruded by gabbroic veinlets that have been highly altered to low-grade or very low-grade assemblages. Contrary to expectations, no rocks from the upper oceanic crust (basaltic lavas or sheeted dikes) were encountered. Protolith assemblages suggest that the peridotites were lherzolites, harzburgites, and dunites.

A 59.34-m-thick, lower Oligocene to Aptian sedimentary succession was cored at Site 1070 (Fig. 9). It is divided into three lithostratigraphic units. Coring commenced in the 20-m-thick Subunit IIC which consists of upward-darkening sequences of calcareous to nannofossil claystone overlain by claystone that were deposited as calcareous turbidites and noncarbonate hemipelagites, probably near or below the CCD. The 39 m of brown claystones in Unit III are interpreted to be the product of slow accumulation of clay in an oxygenated environment on the abyssal plain below the CCD. The manganese content in the claystones and the micronodules also indicate a low rate of deposition. Thin beds of sand near the base may have been deposited as turbidites (one is normally graded) and may be distal equivalents of the high-density turbidites present in Subunit IIIB at Site 897.

The boundary between Units III and IV occurs at the change from brown claystone to nannofossil claystone in Section 7R-2 (17 cm). Matrix-supported sedimentary breccias are interbedded with a 0.4-m-thick nannofossil chalk in Unit IV. The breccias are 3-4 cm thick and normally graded. They were probably deposited as small debris flows or turbidites that transported locally derived material relatively short distances. The chalk is pelagic in origin and was deposited above the CCD. The petrographic features of the calcareous chalks interbedded with the breccias strongly suggest that their constituent calcite grains are largely detrital. The likely source of the calcite clasts is the underlying calcitized basement material.

This site was located in deeper water, farther from turbidite sources, and over a higher basement high than the other sites drilled in this area. Consequently the cores recovered more hemipelagic sediments and less carbonate than in equivalent sections at the other sites, and calcareous fossils were recovered only near the top (Paleogene) and bottom (Lower Cretaceous) portions of the cored sequence. In between, most of lithostratigraphic Unit III is barren. Calcareous benthic foraminifers occur sporadically only between Samples 1070A-1R-1, 43-48 cm and 3R-CC. Agglutinated benthic foraminifers and ichthyoliths occur consistently throughout the cored section from Sample 1070A-1R-1, 43-48 cm through Sample 7R-CC with the exception of Sample 5R-CC, which is barren of foraminifers. The recovery of agglutinated foraminiferal assemblages characteristic of deep water habitats, and the paucity of sediment input of grain size larger than 63 microns suggests that the environment of deposition was probably at bathyal depths during the Aptian.

The basement at Site 1070 is divided into two units, reflecting the dominant petrological features of each (Fig. 9). Basement Unit 1 is found in Cores 7R through 8R and consists of matrix-supported serpentinized peridotite breccias. The clasts consist mainly of serpentinized peridotite with variable amounts of pyroxenes and, in a few cases, of weakly deformed, coarse-grained gabbro. The matrix consists of several generations of calcite, including spherical aggregates (botryoidal calcite). Serpentinization preceded calcite precipitation (serpentinite mesh structures are cut by calcite veins). Abundant "jigsaw" clasts and the transition downward into an incohesive fault gouge suggest a tectonic origin for these breccias. Pervasive calcitization of the matrix and calcite veining indicate that deformation was assisted by fluid flow. The occurrence of tectonosedimentary breccias and gouge on top of serpentinized peridotite, commonly observed during Legs 149 (Site 897) and 173 (Sites 1068 and 1070), is related to a late stage of emplacement of the ultramafic rocks on the seafloor. However, to what extent the breccias are formed by sedimentary processes and/or result from brecciation along fault planes is not yet clear.

The contact between basement Units 1 and 2 consists of a short interval of highly sheared serpentinite over pegmatitic gabbro (i.e., between Cores 1070A-8R-5 and 9R-1). Unit 2, made of igneous (mafic and ultramafic) rocks, starts at 676.2 mbsf in Core 9R and continues through the bottom of Core 14R at 718.8 mbsf. Unit 2 is divided into two subunits: Subunit 2A consists of pegmatitic gabbro and Subunit 2B consists of serpentinized peridotite. Subunit 2A was encountered in Core 9R at a depth of 676 to 686 mbsf. The gabbro shows a clear intrusive contact with the underlying serpentinized peridotites (visible in Core 9R). In the gabbro of Subunit 2A, a weak foliation is observed locally in the core. In thin section, incipient crystal plastic deformation (recrystallization) of plagioclase and amphibole indicates weak heterogeneous ductile deformation. Within Subunit 2B, the serpentinized peridotite is intruded at several locations by 1- to 4-cm-thick, strongly altered, coarse-grained gabbroic dikes. The peridotite displays variable amounts of pyroxene, from pyroxenitic layers in Core 10R (up to 80% pyroxene) to a more dunitic layer in Core 14R (less than 10% pyroxene; see Fig. 7). In general, the peridotites are strongly serpentinized; relict clinopyroxene and spinel are common and small but abundant relicts of olivine are present in the deeper part of the cored section. A high-temperature foliation is only locally marked by the alignment of spinels and pseudomorphs after pyroxenes; it is generally weak and shallowly to moderately inclined (between 9° and 45°). Evidence for possible high-temperature shear deformation is observed in Cores 13R and 14R. These observations suggest that the peridotites were only weakly deformed at high temperatures. The presence of gabbroic veins demonstrates that a later stage of brittle deformation occurred in the mantle rocks during the emplacement of the intrusives.

Results from the pass-through cryogenic magnetometer measurements and discrete sample demagnetization experiments indicate that two or three magnetic polarity reversals may be recorded in the cores. Unfortunately, biostratigraphic data are not available for these cores, except to suggest that these reversals fall in a period between early Oligocene and Late Cretaceous. The most interesting result generated from the preliminary shipboard study is the identification of what appears to be a reversely magnetized zone in the serpentinized peridotites (Subunit 2B). A similar feature found in the serpentinized peridotites recovered from nearby Sites 897 and 899 during Leg 149 suggests that the Iberian peridotites preserved a Cretaceous magnetic record. Further analyses of this magnetic polarity zone, integrated with shore-based geochronologic and isotopic studies, may provide a unique opportunity for dating the tectonic processes that accompanied continental breakup and opening of the North Atlantic.

The poorly consolidated clays, which dominate the sedimentary section, have the lowest bulk densities (1.8 to 2.0 g/cm3) and highest porosities (44% to 54%) measured during Leg 173. The velocities of all sediment samples from above 635 mbsf are less than 2000 m/s, and only two sediment samples in the lower part of lithostratigraphic Unit III have higher velocities of approximately 2200 m/s. The magnetic susceptibility values in Subunit IIC are less than 100 SI units and those in Unit III are between 100 and 400 SI units. There is a 1000 SI units peak of magnetic susceptibility around 649 mbsf in Unit III. Gamma-ray activity is generally between 70 and 80 c/s in Subunit IIC and has a peak of over 110 c/s at 612 mbsf. Gamma-ray activity decreases to between 60 and 70 c/s in Unit III and a peak of over 80 c/s lies at 649 mbsf. All basement samples except one have velocities over 4000 m/s and the maximum value is greater than 7000 m/s. Breccia and serpentinite samples from acoustic basement have bulk densities from 2.3 to 2.5 g/cm3 and porosities from 7% to 17%. Basement rocks have a much higher magnetic susceptibility from a few hundreds of SI units to over 4000 SI units. Gamma-ray values of basement rocks are less than half of those of sediments.

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