Leg 173 Scientific Report


During Leg 173, five sites were drilled and cored in the southern Iberia Abyssal Plain (Sites 1065, 1067, 1068, 1069 and 1070; see Figs. 1 and 2). Site 1066 was the location of a jet-in test for setting a reentry cone that was never used.

Site 1065

Site 1065 lies in a water depth of 4770 m near the northern edge of the Iberia Abyssal Plain. The site is crossed by a single east-west time-migrated seismic reflection profile that shows a large basement high with relief of 1.7 s two way traveltime (TWT; ~2000 m) on its west side, conformably overlain by 0.2-0.35 s TWT of seismically transparent sediment on its landward side. This basement high is the largest of a series of highs on the seismic profile that reduce in relief westward. On time sections, these highs are bounded on their oceanward (west) sides by strong curved reflectors. These reflectors are probably normal faults that were involved in the rifting process, and their presence suggests that the high is a fault block. The basement high and the transparent sediments are overlain by subhorizontally layered post-rift sediments. The principal objective for this site was to identify the nature of the basement rocks and, if continental, to determine the approximate crustal level from which the rocks originated and their petrostructural evolution, to better constrain the modes of crustal thinning and breakup during a rifting episode. Ancillary objectives were to date the overlying pre-rift or synrift sediments and to determine the early subsidence history of the fault block.

Site 1065 was chosen to reach igneous or metamorphic rocks under a tilted fault block with the principal objective of demonstrating that the rocks were continental. While the shallow-water lithologies encountered in the pre-rift/synrift (Middle-Late Jurassic) sediments strongly indicated the presence of underlying continental crust, igneous or metamorphic basement was not reached; the acoustic basement reflection appears to be caused by lithification of Jurassic clays. No clear seismic reflection from the top of the crystalline crust could be discerned beneath the lithified clays.

The lithostratigraphic units used during Leg 173 were numbered so as to be consistent with the unit designations that were defined in this region during Leg 149. The sedimentary section in Hole 1065A (Fig. 3) consists of two lithostratigraphic units: slumped early Miocene nannofossil chalks (Unit II, >58 m thick) overlying Middle to Upper Jurassic clays, claystones, and dolomitic claystones (Unit V, >322 m). The light greenish gray to dark gray thinly bedded and laminated nannofossil chalks and claystones of Unit II were largely deposited from suspension as pelagites and hemipelagites. Pebbles of continental basement lithologies (slate and meta-arenite) and shallow-water limestones occur, and probably originated from the seamounts located ~30 km to the north of Site 1065. The limestones are very similar to Tithonian rocks recovered during Leg 103 from the Galicia Bank margin. Unit V is subdivided into two subunits. Dark greenish gray to medium dark gray soft clay dominates Subunit VA (192.7 m); these clays were remarkably plastic and very slow to drill. Subunit VB (>129.9 m) consists of claystone, some of which is dolomitic.

The structures observed in the cores almost exclusively indicate soft-sediment deformation. Unit II exhibits pervasive slump folds, closely-spaced microfault arrays with both extensional and contractional orientations, and low-angle clay-rich shear zones. Overprinting relations indicate a prolonged history of soft-sediment deformation. Bedding dips measured in the cores are highly variable because of the slumping. Lithostratigraphic Unit V is much less deformed than Unit II. After its deposition, Unit V was tilted into its present, moderately dipping orientation. Bedding dips measured in this unit range between 0° and 65° relative to the core reference frame. The mean dip is 18° with a standard deviation of 10°.

Calcareous nannofossils are moderately to well preserved and abundant in the lower Miocene chalks of lithostratigraphic Unit II, but they are virtually absent in some intercalated siliceous claystones. Relatively undisturbed chalks at the top of Unit II cap the underlying slump complex, providing a minimum date of 16.4 Ma for slump emplacement, based on the absence of middle Miocene planktonic foraminiferal index taxa in the chalk. Small numbers of moderately to well preserved nannofossils provide a Tithonian age for the upper portion of the pre-rift/synrift sediments (Cores 173-1065A-8R to 13R), but only long-ranging taxa are present down to Core 20. A 132-m barren interval separates this core from the lower 33 m of the hole, which contains dissolution-resistant nannofossils that may have been introduced with turbidites. These nannofossils constrain the age of the bottom of the section to Middle to Late Jurassic. Calcareous benthic foraminifers are very rare and occasionally pyritized within these older clastic sediments, which also contain rare, sporadic occurrences of poorly preserved, indeterminate agglutinated foraminifers.

Shipboard magnetostratigraphy within the early Miocene interval is not well defined because of the low core recovery, although there are distinct polarity changes in portions of the cores. The construction of a magnetostratigraphy over this interval is not possible because of the absence of adequate age control, poor recovery, and the disturbance caused by slumps. The Tithonian cores all have normal polarity except for a short reversed interval from 569.7 to 569.9 mbsf. A preliminary calculation of paleolatitude using the inclination data suggests that the drilling site was located about 29°N during the Tithonian, indicating that 11° northward displacement of the region has occurred since the Late Jurassic.

Methane concentrations in headspace gases were very low, varying from 2 to 6 ppm, and concentrations of other hydrocarbons and CO, CO2, and H2S were generally below detection levels. Carbonate contents are relatively high in the chalks of lithostratigraphic Unit II (avg. 68%) and variable in the sediments of Unit V, ranging from values <1% in clay and claystone to around 60% in chalk and dolomitic claystone. Unit II sediments contain only trace amounts of organic carbon, but in Unit V organic carbon contents are quite variable, ranging from 0.1% to 0.85% (avg. 0.4%). The organic matter generally appears to have a marine origin. However, organic C/N values as great as 20 indicate a component of terrestrial material, consistent with the presence of microscopic plant fragments (charcoal) in some sediments.

Between 251.0 and 631.4 mbsf, wet bulk densities range from 1.7 to 2.8 g/cm3, porosities vary between 2 and 60%, and acoustic velocity ranges from 1.7 to 5.7 km/s. Grain densities for all but two samples are almost constant around 2.75 g/cm3. Although bulk densities increase and porosities decrease downward in general, there are two layers associated with lithologic (and most likely seismic) unit boundaries that have particularly low porosities, high bulk densities, and high velocities. One of these layers appears near 330 mbsf, which is just below the interface between lithostratigraphic Unit II and Subunit VA, and the other exists near 500 mbsf, near the boundary between Subunits VA and VB. These layers consist of limestone in which the maximum acoustic velocity is more than two times the velocity in the adjacent sediments.

Logging data correlated well with the lithologic units identified from core observations. A sharp increase of natural gamma ray, a clay-indicator, was observed between Units II and V (300-302 mbsf), as well as a pronounced decrease in the photoelectric effect, an indicator of atomic weight and hence indirectly of sediment type. Neutron porosity was generally high in Unit V, and may be attributed to the high-clay content of the formation and/or to the enlarged borehole. Resistivity increased smoothly with depth. The variable resistivity observed in Subunit VB is probably caused by bands of well-lithified claystone. The calipers revealed that the hole was washed out above 510 mbsf. The hole deviated slightly (1°-2°) in a northwest (350 mbsf) to northeast (600 mbsf) direction. Good-quality Formation MicroScanner (FMS) images were recorded in the lowest part of the hole. Preliminary analysis identified sedimentary layers dipping about 20° toward the east to northeast.

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