The stratigraphic sequence recovered in Hole 898A consists of 340 m of Pleistocene to late Oligocene sediments and sedimentary rocks. Two lithostratigraphic units were recognized that were broadly similar to Units I and II at Site 897. The ages, averaged lithologic compositions, colors, facies and depositional environments, boundary depths, and cored intervals of Units I and II are summarized in Table 2 and Figure 5. Table 3 shows the colors associated with different lithologies.
Subsequent drilling at Site 900 suggested a regional stratigraphic correlation as shown in Figure 6. At Sites 897 through 900 the gross lithostratigraphy consists of a lower carbonate-rich contourite-turbidite-pelagite sequence and an upper turbidite-pelagite sequence. The two sequences contrast sharply in terms of evidence for reworking by contour currents (which is present only in the lower sequence) and in the abundance of siliceous allochems (which are virtually absent in the upper sequence).
Advanced hydraulic piston cores (APC) were taken down to Core 149-898A-14H (132.7 mbsf), and achieved very high recoveries (86%-104%). Subsequent drilling with an extended core barrel (XCB) obtained high recovery for the three cores that penetrated Unit I, but dropped in Unit II to about 70%, with a range from 27% to 101%. One piston core was recovered in Hole 898B and contained sediments similar to Unit I, except for their brown color, which is presumed to reflect the presence of oxidized iron near the sediment/water interface (see "Inorganic Geochemistry" section, this chapter). Sediments from this core have been included in the description of Unit I at Hole 898 A.
Figure 7 shows that the proportion of sand and silt recovered is uniform at about 50% in the upper 12 cores. Below Core 149-898-12H, sand and silt content decrease to <10% at the Unit I/II boundary, from which point it remains relatively constant. Unit I contains greenish gray to greenish black siliciclastic turbidites, mostly capped by light gray nannofossil-rich hemipelagic/pelagic sediments. Subunit IIA consists of a thin (8.8 m) intensely bioturbated brown sequence of silty clay, nannofossil clay, and nannofossil ooze interpreted as pelagic and hemipelagic deposits. Subunit IIB contains light greenish-gray, fine-grained, carbonate-rich turbidites and contourites and darker, carbonate-poor, hemipelagic/pelagic deposits.
Figure 8 (see Table 4) is a plot of biostratigraphic age vs. depth for the sedimentary sequence penetrated at Site 898. Generalized sediment accumulation rates determined from Figure 8 vary from about 90 m/m.y. in Unit I to about 10 m/m.y. in Unit II.
Detrital grains observed in Units I and II at Site 898 are similar to those described for these units at Site 897. Two petrographically distinct clay-rich lithologies occur at this site. "Type 1" claystones and silty claystones contain carbonate in excess of a few percent and show no preferred orientation of the clay particles, which are very fine-grained. "Type 2" clay-rich lithologies contain virtually no carbonate; have coarse, yellow, birefringent clay crystals; and exhibit strong preferred clay orientation (for further description, see "Lithostratigraphy" section, "Site 897" chapter, this volume). A minor difference between Sites 897 and 898 is the occurrence of a few Type 2 claystones in Unit I at Site 898; clay-rich lithologies in Unit I at Site 897 consist entirely of Type 1 claystones.
Cores 149-898A-1H through 149-898A-18X-4, 75 cm, and Core 149-898B-1H
Depth: Hole 898A, 0-163.35 mbsf; Hole 898B, 0-5.40 mbsf
Age: late Pleistocene to late Pliocene
Unit I extends from the seafloor to 163.35 mbsf (Fig. 5). Figure 9 shows that the core recovery for Unit I averages near 30% at Site 897, while recovery at Site 898 approaches 100%. Corresponding to this increase in recovery, the observed percentage of sand in Unit I varies from about 11% at Site 897 to nearly 40% at Site 898. This difference in percentage of sand was interpreted as resulting from differences in coring methods that were employed at the two sites.
Core disturbance ranges from slight to very disturbed. Pronounced sediment flow occurs in most of the first 17 cores and decreases considerably in the remaining cores. Several layers of loosely consolidated sand and silt liquefied during drilling, destroying or disrupting sedimentary structures.
Silt to sand and silty clay to clayey silt dominate in Unit I; minor components include calcareous clay, clay, and nannofossil ooze (Table 2). Table 3 shows the ranges in colors for the different lithologies in Unit I. Much of the unit is greenish gray or dark greenish black. In contrast, light olive colors are common in the more calcareous and less terrigenous detritus-rich Unit II (Table 3).
Unit I consists of turbidites and associated pelagic/hemipelagic sediments. Individual turbidites range in thickness from about 5 cm to more than 1 m; as many as 40 were counted in one core (Core 149-898A-5H). These typically consist of a basal sand (less than 1 to greater than 150cm thick) that passes upward into silty clay, followed by nannofossil clay. Nannofossil ooze locally caps the sequence. The uppermost lithologies of the turbidites are typically bioturbated. Laminations and apparent cross-laminations occur in a few of the turbidite sands (Fig. 10).
Lithologies in the turbidites range from carbonate-poor (silty clays and clays) to carbonate-rich (nannofossil clay). Transitions between these lithologies occur over a span of a few meters and are manifested by color changes (shades of green in terrigenous lithologies and lighter grays in carbonate-rich sediments; Fig. 11).
Applying the classification of Folk (1980), sands and silts of Unit I are subarkoses, arkoses, and lithic arkoses. Grain types in Unit I indicate derivation from a source area were mostly sedimentary, metamorphic, and, possibly, granitic rocks were exposed.
Most quartz grains are monocrystalline and some contain inclusions of rutile, tourmaline, and perhaps sillimanite. Polycrystalline quartz is a minor component, having equant subcrystals with mostly straight boundaries. The feldspar assemblage includes both twinned and untwinned plagioclase, and K-feldspar. Much of the plagioclaseis highly vacuolized, with minor sericite replacement. K-feldspar is principally microcline with very little alteration.
Carbonate rock fragments (CRFs) and metamorphic rock fragments (MRFs) are the main lithic components. CRFs are mostly micritic, but a substantial portion of sparry monocrystals also were observed. A minor component of distinctly rhombic particles, mostly in the silt fraction, is probably dolomite. Other sedimentary rock fragments include minor clay fragments and very rare pieces of chert. MRFs are quartz-mica aggregates derived from low-rank phyllites and schists.
Minor detrital components include abundant micas (muscovite, biotite, and possibly chlorite) that make up to 1% to 2% of the sand-silt fraction. Dense minerals present are mostly ultrastable species, such as zircon and yellow-brown to green tourmaline, with minor epidote, sphene (titanite), apatite, garnet, and green hornblende. Opaque detrital grains include black (magnetite and related minerals) and white (leucoxene) varieties.
Penecontemporaneous calcareous and siliceous marine skeletal debris (generally less than 10%) occurs in the sand/silt fraction, including foraminifers, sponge spicules, diatoms, radiolarians, dinoflagellates, and various nannofossils, including coccoliths and discoasters. In general, a negative correlation exists between the abundance of siliceous allochems and the carbonate content. Sand-sized and coarse silt-sized glauconite is a minor and ubiquitous component. Red brown organic detritus is occasionally noted in the coarse-silt fraction. All the terrigenous clastic material of Unit I is unconsolidated and contains no obvious post-depositional authigenic phases, except for generally minor framboidal pyrite.
Most of the clay-rich lithologies in Unit I contain a significant proportion of clay-sized (<4 µm) carbonate that consists largely of nannofossil debris. Clay minerals examined in smear slides are mostly <1 µm in diameter and show no evidence of crystal elongation or preferred orientation in undisaggregated pieces. Birefringence of small carbonate particles is low, and clay minerals in these rocks cannot be readily distinguished from clay-sized carbonate, except on the basis of color imparted, presumably, from trace amounts of iron oxides and organic matter. Carbonate-bearing clays with very fine clay particles and no evidence of preferred orientation in smear slideshave been designated Type 1. Carbonate contents in some Type 1 clay are low (<10%), but at least a small percentage of nannofossil debris is present.
A few clay-rich rocks in Unit I contain little carbonate and manifest pronounced orientation of the clay minerals within undisaggregated pieces observed in smear slides. These are Type 2 clay-rich lithologies and also larger (up to fine silt-sized) and more highly birefringent (up to first-order yellow) clay crystals.
On the basis of the petrographic similarities with lithologies at Site 897 and the close geographic proximity of the sites, the dominant clay minerals in Unit I are probably illite and kaolinite, with a lesser proportion of smectite (100% expandable).
The setting and processes, which we interpreted to be the deposition of Unit I, are similar to those described in more detail for Unit I, Site 897. The sediments are the product of turbidity current deposition on an abyssal plain. Pelagic sediments accumulated during periods of nonturbidity current deposition.
Compositional changes in both the pelagic and fine-grained terrigenous sediments of Unit I may record fundamental changes in oceanographic/climatic conditions. Increased carbonate content in turbidite lithologies may reflect periods of increased productivity in the water column, reduction in the flux of terrigenous material, or decreased carbonate dissolution. Alternatively, changes in the carbonate content of the turbidites may relate to changes in the location of the sediment source, with carbonate-rich turbidites derived from local submarine highs and more terrigenous turbidites from the continental margin, as suggested by Rothwell et al. (1992) for the Madeira Abyssal Plain.
Section 149-898A-18X-4 at 74 cm to the base of Core 149-898A-36X
Depth: 163.35-339.72 mbsf
Age: middle Miocene to late Oligocene
The major lithologies in Unit II are silty clay/claystone to clay/claystone and nannofossil to calcareous clay/claystone. Minor lithologies are clay/claystone, nannofossil ooze/chalk, and calcareous or foraminifer-rich very fine sandstone to siltstone.
The top of Unit II is defined by a change in facies from the siliciclastic turbidite sequences of Unit I to uniform and/or mottled lithologies lacking significant sand or silt interbeds. The boundary between the units has been placed at the base of the last turbidite sand (at 74 cm in Section 149-898A-18X-4). A gradual change in color from gray to brown takes place across this boundary. The total thickness and age range of Unit II are not known because drilling ceased before reaching the lower boundary. Total recovery of Unit II was 127 m (about 71%).
Two Subunits were recognized in Unit II. Near the boundary between Units I and II, a gradational change occurs from poorly consolidated to more lithified sediments. Beginning with Core 149-898A-18X, cores were split with a saw. Thus, lithologies in Subunit IIB have been described as "stones." The transition between Subunits IIA and IIB is marked also by a distinct increase in the number of thin beds (2-5 cm) of fine sand or silt and carbonate content. In addition, a significant amount (5%-15%) of siliceous microfossils appears for the first time (Fig. 12). The boundary between Subunits IIA and IIB has been placed at the top of this transitional interval, at the uppermost occurrence of laminated silt/sand beds (in Section 149-898A-19X-3 at 137 cm). The transition between Subunits IIA and IIB is characterized also by a gradual color change from brown to gray/green that occurs at the top of Subunit IIB across Intervals 149-898A-19X-3, 137 cm, through 19X-6, 95 cm.
Subunit IIA consists of a mottled and variegated monotonous sequence of dark yellowish brown to very pale orange nannofossil and calcareous clay, clayey nannofossil ooze, and clay/silty clay (Table 2 and Table 3). Lighter shades of brown correspond to higher carbonate contents. Localized concentrations of Fe-rich dark yellowish brown (10YR 4/1) silt or silty clay occur throughout the Subunit. Bioturbation is moderate to intense, with faint irregular mottling; however, no identifiable trace fossils were observed.
Subunit IIB is characterized by an alternation of clay- and carbonate-rich lithologies on a variety of scales. These lithologies are locally mixed by moderate to intense bioturbation. Thin-to-medium thickness, generally upward-darkening color banding in shades of greenish gray is present throughout.
Major lithologies in Subunit IIB are olive-gray silty claystone or clayey siltstone and light greenish nannofossil or calcareous claystone. Minor lithologies include claystone, calcareous fine sandstones to siltstones, and silty nannofossil claystone. White nannofossil/foraminiferal chalk forms about 1% of the total thickness of Subunit IIB. In sandy and silty lithologies, biogenic silica (diatoms, sponge spicules, and radiolarians) is common (up to 5%-15% by volume). Grayish red purple (5RP4/2) and dusky blue (5PB 3/2) laminae or blebs (possibly manganese oxide) are scattered throughout.
The typical sequence of lithologies in Subunit IIB begins with a basal irregular thin bed (up to 5 cm thick, but more commonly 0.5-3 cm thick) of mixed biogenic (calcareous and siliceous) and terrigenous fine sandstone to siltstone (Fig. 13). Parallel lamination, cross lamination, ripple cross lamination (sometimes with flasers), and subtle, normally graded bedding are frequent in carbonate-rich sandstones and siltstones. Foraminifer-rich fine sandstones also show scattered, subtle, reversely-graded bedding, with concentrations of large foraminifer tests on bed tops.
The fine sandstone beds have sharp and flat bases and sharp to gradational tops (Fig. 14). Where tops are gradational, the transition from fine calcareous sandstone to the overlying nannofossil claystone is characterized by wavy, parallel, or lenticular alternating laminae of both lithologies, as well as by burrow mottling (Fig. 14). These sandy or silty beds make up about 5% of the total thickness of Subunit IIB.
This basal silt- and sand-rich lithology is overlain by color-banded (8-15 cm thick) clay-rich lithologies. Color variations correspond strongly to variations in the carbonate content (mostly nannofossils). Light-colored nannofossil claystones to clayey nannofossil chalks having varying contents of silt occur near the bases of the upward-darkening sequences. Darker colors correspond to the overlying silty claystones and claystones. The boundaries of colored bands have been locally disturbed by pervasive bioturbation.
Burrows are concentrated (or more clearly visible) in the light-colored, carbonate-rich lithologies in the lower part of each individual sequence. In places, intensive burrowing has modified the original depositional contacts within a sequence and, usually, the burrowing extends down the sequence to the basal calcareous sand bed. The most common trace fossils are Planolites, and Chondrites, with less abundant and isolated Zoophycos and Skolithos.
In addition to the dominant upward-darkening sequences, upward-lightening sequences, and intensely bioturbated, sharp-based, white nannofossil chalk beds also are seen.
Sand- and silt-sized detritus in Unit II includes components essentially similar to the assemblage observed in Unit I. Unit II also contains clay-rich lithologies that are similar to those in Unit I, except that the Type 2 (oriented, carbonate-poor) clay-rich lithologies are a more persistent and volumetrically significant part of the lithologic assemblage. Both claystone types are present in repeated interbeds and in various shades of browns and green grays.
Based on the petrographic similarities with lithologies at Site 897 and the geographic proximity of the sites, the dominant clay minerals in Unit II are probably similar to those in Unit I (i.e., illite and kaolinite with a lesser proportion of expandable clay).
The homogeneous lithologies in Subunit IIA are interpreted as a pelagic and hemipelagic facies. Depositional processes inferred for these facies include continuous, slow accumulation by settling through the water column and the absence of turbidity or other bottom currents.
Ubiquitous bioturbation in the sediments of Subunit IIA suggests deposition under conditions of normal oxidation. The relatively high content of biogenic carbonate indicates that deposition probably took place above the CCD. The position of the Subunit, sandwiched between the terrigenous sandy turbidites of Unit I and the distal, muddy, current-deposited facies of Subunit IIB, suggests deposition in an abyssal plain setting.
In Subunit IIB, upward-darkening alternations of carbonate-rich and relatively carbonate-poor sediments containing biogenic siliceous material contrast with the clearly turbidite-related lithologic association so characteristic of Unit I. Thin silty sand and silt intervals at the bases of many of the upward-darkening sequences lack the clear normal grading typical of turbidites. Many of these silty sands and silts contain small-scale cross stratification and parallel lamination, which indicates current activity (Fig. 13, Fig. 14, Fig. 15). Some exhibit inverse grading or have sharp tops as well as sharp bases and are overlain by "lag deposits" of large foraminifers. All these features point to reworking by contour currents, as described by Stow and Piper (1984). It is possible that some of the homogeneous, carbonate-poor terrigenous silty claystones and claystones are mud turbidites. In general, the ubiquitous presence of minor pelagic intervals (nannofossil claystone to nannofossil chalk) suggests that deposition of Subunit IIB occurred above the CCD.
The combination of possible turbidites and contourites observed in this Subunit is not unexpected. Possible biostratigraphic hiatuses (see "Biostratigraphy" section, this chapter), relatively low sediment accumulation rate, and the physiography of the Cenozoic Iberian margin are all factors consistent with contourite development (see Comas and Maldonado, 1988).