HOLE 897D

Date occupied: 9 April 1993

Date departed: 17 April 1993

Time on hole: 7 days, 23 hr, 15 min

Position: 40°50.31'N, 12°28.51'W

Bottom felt (drill-pipe measurement from rig floor, m): 5327.0

Distance between rig floor and sea level (m): 11.23

Water depth (drill-pipe measurement from sea level, m): 5315.8

Total depth (from rig floor, m): 6164.20

Penetration (m): 837.20

Number of cores (including cores having with no recovery): 25

Total length of cored section (m): 241.20

Total core recovered (m): 117.94

Core recovery (%): 48.75

Oldest sediment cored:

Depth (mbsf): 693.80
Nature: nannofossil-foraminifer chalk, claystone
Age: Early Cretaceous
Measured velocity (km/s): 2.07

Basement:

Depth (mbsf): 837.20
Nature: serpentinized peridotite
Measured velocity (km/s): 3.0-5.4

Principal results: Site 897 is situated in the Iberia Abyssal Plain over a north-south basement ridge within the ocean/continent transition (OCT) zone. Geophysical modeling had predicted that the ridge lay at, or close to, the ocean/continent boundary and might consist of serpentinized peridotite. The site is one of a transect across the OCT designed to study the petrological changes in the basement rocks within the OCT as a means of identifying the processes that accompanied continental break-up and the onset of steady-state seafloor spreading. Cores were obtained from three holes that penetrated up to 694 m of Pleistocene to Early Cretaceous age sediments and from two holes that penetrated up to 143 m of basement.

1. The basement is composed of serpentinized, relatively undepleted peridotite that originated in the upper mantle and was exposed at the seafloor during, and for several tens of m.y. after, the time of continental break-up.
2. A unit containing late Hauterivian(?), early Barremian to late Aptian sediments immediately overlies basement and contains fragments of peridotite and continental basement rocks. The unit was deposited during the Early Cretaceous by a series of mass flows. The composition of the sediments suggest that continental basement rocks were located upslope, or a few tens of meters downslope, of the site.
3. A significant depositional hiatus starting in the middle Miocene, correlatable with a regional angular unconformity on seismic reflection profiles, may be related to northwest-southeast compression on this margin during a Compressional phase in the Betic Mountains in southern Spain and structural inversion in the Lusitanian Basin of Portugal.

Four lithologic units have been identified at Site 897:

1. Unit I (0-292.0 mbsf) is a Pleistocene to early Pliocene silty clay to clayey silt with nannofossil clay with graded silt and fine sand beds. The unit mainly consists of terrigenous turbidites.
2. Subunit IIA (292.0-301.2 mbsf) is an early Pliocene to late Miocene nannofossil claystone with claystone and nannofossil silty claystone of turbiditic and hemipelagic/pelagic origin.
   Subunit IIB (301.2-359.8 mbsf) is a late to early Miocene calcareous claystone and claystone with silty claystone and clayey siltstone. The Subunit mainly consists of calcareous turbidites with significant reworking by contour currents.
   Subunit IIC (359.8-619.7 mbsf in Hole 897C; base at 622.9 mbsf in Hole 897D) is an early Miocene to middle Eocene silty claystone to clayey siltstone with calcareous claystone. The subunit consists mainly of calcareous turbidites with significant reworking by contour currents.
3. Subunit IIIA (619.7-639.4 mbsf in Hole 897C; 622.9-645.2 mbsf in Hole 897D) is an unfossiliferous claystone. The subunit consists of a pelagic/hemipelagic facies deposited below the carbonate compensation depth (CCD).
   Subunit IIIB (639.4-648.7 mbsf in Hole 897C; 645.2-655.2 mbsf in Hole 897D) is an unfossiliferous clayey conglomerate and sandy silty claystone with clayey sandstone.
4. Unit IV (648.7-677.5 mbsf in Hole 897C; 655.2-693.8 mbsf in Hole 897D) consists of late Aptian to Hauterivian sandstone, dolomite, limestone, calcareous claystone with peridotite clasts and megaclasts. The unit is a mass-flow deposit.

The sedimentary section provided a discontinuous fossil record from the Pleistocene through Early Cretaceous. Calcareous nannofossils are generally present, except in Unit III. Planktonic and benthic foraminifers are abundant to common in the Pleistocene and upper Pliocene deposits, but rare to absent throughout most of the remaining cored interval. One hiatus was observed in the upper Pliocene. Two unconformities in the Miocene together represent a 10.5-Ma hiatus from middle Miocene to late Miocene. Although the age of Unit III is uncertain, it is likely that an earlier hiatus in deposition occurred from the Early Cretaceous to the middle to late Eocene.

The mass-flow deposit immediately above the basement, containing basement blocks, was emplaced during late Hauterivian(?), early Barremian and late Aptian times. Therefore, the peridotite was exposed at the seafloor and contributed clasts to the mass flows during the Early Cretaceous.

Holes 897C and 897D penetrated 67 m and 143 m, respectively, of basement rock. The entire basement section consists of peridotite that has been almost completely serpentinized and partially brecciated during and after serpentinization. About 90% of the peridotite is undifferentiated harzburgite or lherzolite whose original composition was 70%-80% olivine, 15%-20% pyroxene, and 1%-2% spinel. These rocks are heterogeneous and range from pyroxene-rich peridotite to dunite. The remaining 10% of the peridotite is plagioclase- and spinel-bearing and was originally composed of 50%-70% olivine, 20%-30% pyroxene, 15% plagioclase, and 1%-5% spinel. The coexistence of plagioclase and spinel suggests that these rocks last equilibrated at low pressure, (900-1000 MPa or about 30 km depth). The wide variety of peridotite types and the locally high proportion of plagioclase (up to 40%), suggest that the peridotite may have experienced some melting and even magma mobility. The brecciation ranges from pervasive fracturing and serpentine and carbonate veining to the formation of gravel-sized serpentinized peridotite clasts embedded in a carbonate and serpentine matrix. In some cases, the brecciation shows a well-developed foliation associated with a late-stage shear deformation event. While the peridotites have experienced late, low-temperature deformation, they show almost no signs of high-temperature ductile deformation.

Acoustic formations 1A, 1B, and 2 have been widely recognized in multichannel seismic reflection profiles on the western Iberia margin and have been locally dated by previous drilling. At Site 897, acoustic formation boundaries 1A/1B and 1B/2 correlate with the middle to late Miocene hiatus at 325 mbsf and with the top of middle Eocene claystone and chalk at about 590 mbsf, respectively.

Several tentative magnetostratigraphic correlations have been identified in the Pliocene, and one each in the middle Oligocene and Maastrichtian. Magnetic susceptibility correlates particularly well on a fine scale with the alternating sand and clay units of the terrigenous turbidite sequences in Unit I. Magnetic susceptibility generally is higher in the serpentinized peridotite basement than in the sediments, but decreases with increasing peridotite alteration.

In-situ temperature measurements indicate that the vertical temperature gradient between the seafloor and 215 mbsf is about 43°C/km and that the vertical conductive heat flow is about 54 mW/m² .

Physical property measurements in Units I and II show a steady increase in density, seismic velocity, formation factor, and thermal conductivity, and a concomitant decrease in porosity with depth. Those in Units III and IV are much more variable. In the serpentinized peridotite, density and velocity vary according to the degree of alteration visible in hand specimens and range from 2.3 to 2.5 g/cm³ and 2.8 to 7.1 km/s.

Interstitial-water samples were obtained from Unit I through Subunit IIIA (26-636 mbsf). The pore-water chemistry (sulfate, alkalinity, ammonia, iron, manganese, calcium, magnesium, strontium, potassium, silica, chloride, and sodium) reflects the rapid deposition of the Pliocene-Pleistocene turbidites and the relatively slower deposition of the earlier sediments. The principal result is the surprisingly high sulfate concentration (up to 18 mM) in the sediments below Unit I and Subunit IIA. This is attributed to a low sedimentation rate, low permeability, and an absence of reactive carbon.

Profiles of carbonate content vs. depth reflect a history of generally low biological productivity and deposition of hemipelagic sediment below the CCD, combined with delivery by turbidites of carbonate-rich material initially deposited above the CCD. Elevated organic carbon is found in Units I and IV (0.6 and 0.9%, respectively); Unit IV appears to contain organic material mostly derived from land plant detritus. Relatively high concentrations of biogenic methane were encountered in headspace gas analyses of lithostratigraphic Unit I, but were essentially absent from all deeper sections.