The composite sedimentary section recovered at Site 897 (Holes 897A, 897C, and 897D) includes nearly 700 m of Pleistocene to Early Cretaceous sediments and sedimentary rocks, overlying serpentinized peridotite of unknown age (Fig. 8). Pleistocene sediments were recovered at Hole 897A (0-55.2 mbsf), a Pleistocene to Early Cretaceous sedimentary sequence at Hole 897C (49.9-677.5 mbsf), and a middle Eocene to Early Cretaceous sedimentary sequence at Hole 897D (596.0-693.8 m). The lithostratigraphic succession at Site 897 is divided into four lithostratigraphic units on the basis of the degree of lithification and changes in lithology.
Subsequent drilling at Site 900 suggested the regional stratigraphic correlation shown in Figure 9. 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 (virtually absent in the upper sequence).
Siliciclastic turbidites facies occur in Unit I and subunit IIA and carbonate-rich turbidites reworked by contour currents in Subunits IIB and IIC. The hemipelagic/pelagic tops of the turbidite sequences are composed of very fine-grained carbonate sediments in Unit I, but are carbonate-poor in Unit II. Subunit IIB is characterized by the presence of brown sediments, whereas Subunits IIA and IIC contain a mixture of gray, green, and brown sediments.
Unit III is exclusively siliciclastic in composition, except for carbonate pebbles and granules in the conglomerates in the lower part of Subunit IIIB. Subunit IIIA is composed entirely of brown terrigenous clay, whereas Subunit IIIB consists of an upward-fining sequence of pebble and gravel conglomerates, coarse sandstones, and sandstones and silty claystones.
Unit IV rests on basement at Holes 897C and 897D and comprises several intervals of serpentinized peridotite (see "Igneous and Metamorphic Petrology and Geochemistry" section, this chapter) between intervals of siliciclastic and carbonate lithologies.
The ages, average lithologic compositions, colors, facies, and depositional environments, boundary depths, and cored intervals of the units are summarized in Table 2. In Table 3, detailed information is given about color variations among lithologies in Units I and II. Colors in Unit III are highly variable and are described in the text; colors in Unit IV are documented in Table 4.
Figure 10 is a plot of age vs. depth data from Table 5 for the sedimentary sequence penetrated at Site 897. Generalized sediment accumulation rates obtained from Figure 10 show variations that exceed an order of magnitude across the units identified at this site: Unit I, 55 m/Ma; Unit II, 8 m/Ma; combined Units III and IV, 0.8 m/Ma.Cores 149-897A-1R through -6R; Cores 149-897C-1R through -26R, 50 cm
Depth: 0-55.2 mbsf (Hole 897A) and 49.9-292.0 mbsf (Hole 897C)
Age: Pleistocene to early Pliocene
Unit I extends from the seafloor to 292.0 mbsf, where a distinct change in sediment stiffness is seen. All cores in Unit I were split using a wire. Core recovery in the upper part was 10%-25%, but this improved below 150 mbsf to 25%-50%. Increased recovery with depth was accompanied by decreased drilling disturbance in the cores. Recognition of vertical changes in grain sizes in the uppermost cores (149-897A-1R through -5R, and 149-897C-1R) were hampered by extreme drilling disturbance and poor recovery.
Unit I consists of turbidite sequences and associated hemipelagic/pelagic sediments. Individual turbidite-to-pelagite cycles range in thickness from 5 to 60 cm and display a sequence of characteristic textural changes (Fig. 11). A basal, normally graded sand-to-silt interval (1-10 cm thick) usually overlies a sharp erosional contact with underlying fine-grained sediments and grades upwards into a clay-rich and nanno-fossil-enriched interval, which, locally, then grades into nannofossil ooze. In some individual sequences, the sand-to-silt interval is missing, either because of nondeposition or loss during drilling. Within individual turbidite/pelagite cycles, some lithologic boundaries are highly gradational as a result of depositional processes, bioturbation, and drilling disturbance.
Lighter colors generally are correlated with higher carbonate content. Nannofossil ooze, nannofossil clay, and calcareous clay are mostly very light and medium grays to greenish grays. A few samples of these lithologies range into darker colors, such as greenish black. Nannofossil ooze contains up to 78% carbonate (see "Organic Geochemistry" section, this chapter). Nannofossil clay grades into calcareous clay, depending on the preservation state of the nannofossils and the presence of other carbonate grains, such as foraminifers and carbonate rock fragments. Sedimentary structures in the Unit I nannofossil-rich sediments are limited to minor lamination and mottling from bioturbation.
Darker colors (olive grays, brownish grays, and so forth) are observed in calcareous and nannofossil silty clay to clayey silt and in silty clay to clayey silt. This may reflect higher contents of organic matter or absence of carbonate (e.g., Core 149-897C-23R). Lamination in some silty clay intervals (e.g., in Core 149-897C-8R) may be drilling-induced.
Beds and laminae of silt to fine sand are found throughout Unit I; medium sand is limited to Cores 149-897A-1R and -3R. Silt-to-sand intervals have colors similar to the silty clay to clayey silts. As with the finer-grained lithologies, the darker colors of the silt and sand beds may be related to higher contents of organic matter. Smear-slide analyses show that silt-sized organic matter locally reaches values of 30% (e.g., Core 149-897C-23R), in contrast to the generally low organic content of this unit as a whole (see "Organic Geochemistry" section, this chapter). Pyritized organic material is present (e.g., Core 149-897C-23R) and pyrite-filled burrows are found in Cores 149-897C-1R, -5R, -6R, and -16R.
Silt and sand in Unit I are arkosic, being dominated by quartz and feldspar. Applying the classification of Folk (1980), the sands are subarkoses, arkoses, and lithic arkoses. Sand and silt compositions in Unit I, as described below, indicate derivation from a source area that exposed sedimentary, metamorphic, and, possibly, granitic rocks.
Quartz grains are mostly monocrystalline, and some contain inclusions of rutile, tourmaline, and perhaps sillimanite. Polycrystalline quartz is a minor component, characterized by equant subcrystals with mostly straight boundaries. The feldspar assemblage includes both twinned and untwinned plagioclase and K-feldspar. Much of the plagioclase is highly vacuolized, with minor sericite replacement. Much of the K-feldspar is microcline, which shows little alteration.
Carbonate rock fragments (CRFs) and metamorphic rock fragments (MRFs) occur. CRFs are mostly micritic, but a substantial portion of sparry monocrystals also are observed. A minor component of distinctly rhombic particles, mostly in the silt fraction, is probably dolomite. Other sedimentary rock fragments include minor claystone fragments and rare pieces of chert. MRFs are quartz-mica aggregates that 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.
Most samples contain a proportion (generally less than 10%) of penecontemporaneous calcareous and siliceous marine skeletal debris, including foraminifers, sponge spicules, diatoms, radiolarians, dinoflagellates, and various nannofossils. Locally, siliceous allochems have been partially replaced by pyrite. Sand-sized and coarse silt-sized glauconite is a minor and ubiquitous component. Red-brown organic detritus can be noted occasionally in the coarse silt fraction. All terrigenous clastic materials of Unit I are unconsolidated and contain no obvious post-depositional authigenic phases, except for generally minor framboidal pyrite and partial replacement of wood (e.g., smear slides from Core 149-897C-23R).
Most of the clay-rich lithologies in Unit I contain a significant proportion of clay-sized (<4 m carbonate that consists largely of nannofossil debris. The clay-rich lithologies are mostly unconsolidated and disperse readily in distilled water. Clay minerals examined in smear slides are mostly <l m in diameter and exhibit no evidence of the crystal elongation and preferred orientation observed in the clay minerals of deeper units (see below). Birefringence of these small 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. Clay and claystone having the above-described petrographic characteristics are designated "Type 1." Carbonate contents in Type 1 claystones are low (<10%), but at least a small percentage of nannofossil debris is present.
Cursory XRD examination of clay minerals in Unit I suggests the presence of detrital illite, kaolinite, and an expandable clay.
During the Pliocene-Pleistocene, deposition on the Iberia Abyssal Plain was dominated by the repeated emplacement of thin- to medium-bedded, fine-grained turbidites. Much of the sediment was probably funneled through submarine canyons that cross the Portuguese continental margin. Unit I includes sediments laid down by both turbidity currents and pelagic to hemipelagic processes.
The structureless to laminated basal sand and silt layers in the turbidites correspond to the Tc-d division of the classic Bouma sequence (Bouma, 1962); however, the Ta-b interval is absent. Clay-rich intervals are bioturbated to laminated, contain significant amounts of reworked nannofossils (see "Biostratigraphy" section, this chapter), and correspond to the turbiditic mud division (Te 1-3) of Piper (1978); this division represents deposition from the low density tail of the turbidity current. The bioturbated nannofossil clay-to-ooze intervals are pelagic deposits. However, visually distinguishing the Te facies from the pelagic interval is difficult because the sediments are fine-grained and have been somewhat mixed by bioturbation and drilling disturbance.
The thicker (up to 50 cm) loosely consolidated and normally graded sand units in the uppermost part of Unit I may be a product of differential grain settling during coring. Low recovery in these sandy intervals precludes any definitive interpretation, but they may possibly have been deposited from one or several turbidity currents, or they may represent sand concentrated by winnowing and reworking.
Structureless, fine-grained units up to 1.5 m thick also occur at several levels in the unit. Given the presence of turbidites, the overall relatively high sedimentation rates (Fig. 10) and the typically low sedimentation rates of carbonate in the deep ocean, it is unlikely that these are pelagic units that accumulated during a prolonged lull in terrigenous sedimentation. Therefore, these intervals may be either a single, thick, massive mud turbidite or a series of amalgamated mud turbidites.
Carbonate-rich layers of well-preserved benthic foraminifers and calcareous nannofossils at the tops of the turbidites suggest deposition above the CCD, although relatively rapid deposition and burial might have aided carbonate preservation.
In Unit I, a maximum 32 turbidites occur in a single core (149-897C-14R). Turbidite frequency in Unit I ranges from about two to seven per meter of core, which suggests that relatively little time elapsed between successive turbidity flows, consistent with the relatively high sediment accumulation rate observed for Unit I. Trace fossils in the upper parts of the turbidites indicate that periods between successive flows were sufficiently long to enable burrowing organisms to populate the substrate and that oxygen levels in bottom waters were at least dysaerobic.
Cores 149-897C-26R-1, 50 cm, through -60R-1, 0 cm
Cores 149-897D-1R-1, 0 cm, through -3R-5, 38 cm
Depth: 292.0-619.7 mbsf (Hole 897C) and 596.0-622.9 mbsf (Hole 897D)
Age: late Miocene to middle Eocene
In addition to being defined by the first occurrence of carbonate-rich turbidites, the top of Unit II also is marked by a distinct increase in sediment consolidation in Core 149-897C-26R. Soon after passing this boundary (Core 149-897C-29R), a saw was routinely used to split the cores; thus, these are described as "stones," as opposed to "sediments." In addition, a transitional change from gray/green to brown colors occurs at the Unit I/Unit II boundary.
Core recovery was relatively low in the top 20 m of this unit, but below this level, it increased dramatically, averaging more than 50%. Cores are slightly to moderately fractured (biscuited) throughout.
This subunit occurs within a single core (149-897C-26R) in which the first partially lithified sediments occur which are transitional in character between those of Unit I and the remainder of Unit II.
Bedding contacts between the major lithologies in Subunit IIA (nannofossil claystone, nannofossil silty claystone, and claystone) are gradational as a result of the combined effects of drilling disturbance ("biscuits"), splitting the core with a wire, and moderate bioturbation. Three thin (up to 4 cm), graded-to-laminated beds of silty sand are present in the core. Color of the fine-grained lithologies changes down the core from green to brown. The major lithologies are similar to those described below in Subunit IIB; however, the silty sands are more similar in color (dark olive black [5Y 2/1]) to those in Unit I.
The lower part of Unit II is divided into two subunits (IIB and IIC) on the basis of the proportion of claystone vs. coarser-grained lithologies, such as silty claystone, siltstone, and sandstone.
Subunit IIB consists predominantly of calcareous/nannofossil clay-stone and claystone, with minor silty claystone, nannofossil chalk, and clayey-to-silty/fine sand layers occurring as repetitive turbidite/pelagic sequences. These show a basal calcareous sand-to-silt interval grading upward into nannofossil claystone that is sometimes laminated. In turn, the nannofossil claystone is overlain by silty claystone or claystone that likely represents pelagic sedimentation.
The major component of Subunit IIB is calcareous claystone that exhibits a wide range of colors, mostly browns and grays. The subunit is characterized by intensive color mottling caused by bioturbation. The ichnofauna includes Zoophycos, Chondrites, and Planolites. Apart from bioturbation, the calcareous claystones are structureless.
The minor lithologies include silty claystone and claystone, nannofossil claystone, nannofossil silty claystone, and nannofossil chalk. Lighter shades of gray correlate with higher carbonate contents. The minor lithologies are extensively bioturbated, and the nannofossil silty claystone and nannofossil chalk are locally laminated (Fig. 12). The distinct coloration of several thin, reddish purple laminae in Section 149-897C-30R-1 suggests manganese enrichment.
Layers of calcareous silt-to-fine sand occur in Cores 149-897C-30R and -31R (Fig. 12) and range from 10 to 20 cm thick. These coarse basal units typically are greenish gray and grade upward into nannofossil claystones of the same color.
The top of Subunit IIC occurs in Core 49-897C-33R-1 at 65 cm, at the first occurrence of a dominantly yellowish brown claystone succession. This color is in marked contrast to the greenish gray claystones of Subunit IIB. The base of the subunit is defined as the last occurrence of greenish gray claystones interbedded with some brown claystones at the top of 149-897C-60R and at -897D-3R-5, 38 cm. Repeated darkening-upward sequences also occur in Subunit IIC.
The subunit is dominated (about 90%) by claystones and silty claystones to clayey siltstone. These lithologies exhibit a variety of colors, mostly browns and grays.
Carbonate-rich lithologies also are important components of Subunit IIC. These are lighter colored than the fine-grained siliciclastic lithologies and are mostly lighter grays. Carbonate-rich lithologies include calcareous clay, claystone with nannofossils, nannofossil silty claystone, nannofossil claystone, nannofossil foraminiferal chalk, and nannofossil chalk.
The minor siliciclastic lithologies include siltstone and silty sand that contain mixtures of siliciclastic and calcareous grains. Light-colored nannofossil claystones and chalks also occur only sporadically and make up less than 1% of the section measured in Subunit IIC.
Two intervals contain brown claystones and silty claystones: Interval 149-897C-47R-3, 90 cm, to the base of Core 149-897C-52R (thickness, 48.9 m), and Interval 149-897C-55R-1, 0 cm, to the base of Core 149-897C-59R (thickness, 43.3 m).
All the cores display a series of sharp-based, upward-darkening sequences between 5 and 50 cm thick. In the brownish intervals, these show the following sequence of lithologies, numbered from bottom (1) to top (4):
| 4 Massive silty claystone | Dark yellowish brown (10YR 4/2) | |
| 3 Nannofossil silty claystone | Pale yellowish brown (10YR 6/2) | |
| 2 Nannofossil silty claystone | Greenish gray (5GY 6/1) | |
| 1 Silty fine sandstone or siltstone | Greenish gray (5GY 5/1) or light gray (N7) |
Nannofossil silty claystone and massive silty claystone (Lithologies 1 and 2) are mixed by burrowing, and Zoophycos, Chondrites, and Planolites are commonly observed within the transition between them. This ichnofauna also occurs in silty fine sandstone and nannofossil silty claystone. The massive nature of Lithology 4 was probably caused by intense bioturbation. Nannofossil silty claystone sometimes shows planar cross lamination, and Zoophycos may occur toward the top of it.
In the nonbrownish intervals, the typical lithological sequence (Fig. 13A) is as follows, numbered from bottom (1) to top (3):
| 3 Claystone or silty claystone | Olive gray (5Y 4/10 or dark greenish gray (5GY 4/1) | |
| 2 Silty claystone and/or nannofossil silty claystone | Greenish gray (5GY 5/1-6/1) | |
| 1 Silty fine sandstone | Dark greenish gray (5GY4/1) |
Burrow mixing occurs commonly between silty claystone and/or nannofossil claystone and claystone or silty claystone. Planolites, Chondrites, and Zoophycos are common in these intervals. In some cases, Planolites and Chondrites mix Lithologies 1 and 2 into the underlying sequence.
Silty or sandy bases are not always present in either type of upward-darkening sequences (Fig. 13B); however, in some intervals, silty fine sandstone forms the bases of normally graded units, particularly in Cores 149-897C-30R, -40R, -42R, -43R, -50R, -52R, -53R, -54R, and -59R.
The basal interval of some of the sequences is composed of medium sand. It is possible that the sandier basal units are under-represented in the cores and have been lost between the drilling biscuits. A higher proportion of sandy intervals was recovered in Hole 897D at the level equivalent to Core 149-897C-59R, possibly the result of less drilling disturbance.
Sandy layers with possible cross-stratification and sharp tops attest to likely reworking by contour currents (Fig. 14A–14B).
Sand- and silt-sized detritus in Unit II includes components essentially similar to the assemblage observed in Unit I. Some of the better examples of MRFs are not generally observed in Unit II because of the generally finer grain size of the sand, but the general characteristics of the rock fragment assemblage are the same as those in Unit I.
Subunit IIA contains clay-rich lithologies that are similar in most respects to those in Unit I, except for an overall lower carbonate content. The smear slide at Sample 149-897C-27R-CC, 8 cm, in Subunit IIIB marks the appearance of a distinctive clay-rich lithology that contains virtually no carbonate. This lithology is designated as "Type 2," in contrast to carbonate-rich claystone varieties called "Type 1" in Unit I.
Clay minerals in the Type 2 claystones and silty claystones are characterized by a larger range of crystal sizes that range up to fine silt. The more coarsely crystalline part of the clay assemblage has higher birefringence (first-order yellow to orange) than that observed in the finer-grained clays in the carbonate-rich claystones of Unit I. Many of the coarser clay minerals are also distinctly elongated, forming small fibers approximately 1 m wide and up to 20 m long. Silt-sized particles associated with this coarser clay assemblage are typically coated by 0.5 to 1 m rims of yellow birefringent clay. Small (<10 m), near-isotropic flakes of mica(?) coated in this manner produce a distinctive texture in many of the Type 2 claystone smear slides. The minor amount of carbonate that occurs in these claystones is mostly in the form of 10- to 30-m-thick, rhomb-shaped crystals that may be detrital dolomite. In smear slides, pieces of claystone that have not disaggregated contain coarse yellow birefringent clay minerals having a high degree of preferred orientation.
Both claystone types are present in repeated interbeds and in various shades of browns and green-grays, but in Subunit IIC, Type 1 claystones are lighter in color (mostly greens to grays), whereas Type 2 claystones are red brown (see previous discussion of the two types of upward-darkening sequences).
Cursory XRD examination of clay minerals in Unit II suggests that both types of claystone contain similar clay mineral assemblages, including illite, kaolinite, and a highly expandable clay.
The depositional processes for Subunit IIA are similar to those outlined for Unit I, but few of the turbidity flows deposited sand or silt.
Calcareous turbidite facies, according to the classical Bouma sequences (Bouma, 1962), dominate Subunit IIB. These sequences exhibit facies Td through Te, plus a hemipelagic/pelagic interval. The carbonate content of each turbidite in Subunit IIB decreases upward. The relatively high carbonate content of the basal intervals of each individual turbidite sequence reflects the presence of CRFs and locally abundant foraminifers within the silt/sand fraction. The upward decrease in carbonate content of the upper hemipelagic/pelagic parts of the sequences may have been caused by:
These causes have been linked to cyclical variations in climatic and oceanographic conditions to explain the repeated occurrence of calcareous oozes and terrigenous deposits on the western Iberian margin at DSDP Site 398 (Maldonado, 1976) and ODP Site 637 (Comas and Maldonado, 1988). One or more of the above effects may have had a part in producing the characteristic turbidites of Subunit IIB at Site 897.
Turbidite, and possibly contourite, facies dominate the sediments of Subunit IIC. Textural and compositional changes within some graded or upward-darkening sequences are similar to those described in Subunit IIB. Individual sequences consist of a basal siltstone or fine sandstone interval that passes upward into silty claystone, claystone, or nannofossil claystone (Td to Te facies; Bouma, 1962). However, some intervals of calcareous siltstone to fine sandstone in Subunit IIC contain features (sharp bases and tops, parallel and cross lamination in siltstones, and parallel silty laminations in claystones) that might reflect deposition by contour currents. Comas and Maldonado (1988) attributed similar facies of the Pliocene-Pleistocene sequence at Site 637 to winnowing and reworking of the substrate by bottom currents. Additional detailed work will be necessary to evaluate the roles of contour currents vs. turbidity currents during the accumulation of Subunit IIC.
Sequences in the lower half of Subunit IIC exhibit upward compositional trends similar to those in Subunit IIB and so also are interpreted as turbidites and carbonate-poor hemipelagic/pelagic sediments that resulted from a combination of conditions, as described for Subunit IIB.
Cores 149-897C-60R-1, 0 cm, to the base of -62R
Cores 149-897D-3R-5, 38 cm, to the base of -6R
Depth: 619.7-648.7 mbsf (Hole 897C) and 622.9-655.2 mbsf (Hole 897D)
Age: uncertain
Unit III contains only a few fossils, all within detrital clasts. The age of this unit can be constrained only on the basis of the ages of the units above and below: middle Eocene to post-Aptian (see "Biostratigraphy" section, this chapter).
The top of Unit III is defined by an abrupt change from the upward-darkening sequences of Unit II to a massive brown claystone to silty claystone (Table 2). In Hole 897C, the change in lithology and color between Units II and III may not have been recovered, as it occurs at a core boundary (the top of Core 149-897C-60R). The contact was recovered at Hole 897D in Core 149-897D-3R, where variegated claystone is overlain by a graded bed of clayey medium-to-coarse sandstone; the contact is placed at the base of this sandstone.
The major lithologies of Unit III are claystone, silty claystone, and sandy silty claystone, with lesser proportions of sandy claystone, matrix-supported sand with dispersed granule-sized lithic clasts, and polymictic, matrix-supported, granule-to-pebble conglomerate.
The unit is divided into two subunits at a lithologic change from massive claystone and silty claystone to coarser-grained clastic sediment. The boundary between Subunits IIIA and IIIB is gradational. The uppermost part of Subunit IIIB (Sections 149-897C-62R-1 and -62R-2) consists of claystone and silty claystone (similar to that of Subunit IIIA) interbedded with thin beds of sandy claystone and clayey sandstone. The Subunit IIIA/IIIB boundary was arbitrarily placed at the top of the first sandstone interval.
Subunit IIIA consists of a monotonous sequence of claystone and minor silty claystone. Colors range from dark reddish brown (10R 5/1) to moderate brown (5YR 3/4) to light brown (5Y 5/6). Round-to-elongate, greenish gray (5GY 6/1) reduction spots are scattered throughout this subunit. Bedding is poorly defined or absent. The clay shows vague color-banding, and bioturbation is not apparent. The core is moderately disturbed and exhibits drilling biscuits.
Subunit IIIB includes sandy-to-silty claystone; fine, medium, and coarse, poorly sorted, matrix-rich sandstones; and matrix-supported, poorly cemented, granule- to pebble-sized, poorly sorted conglomerate. At both Holes 897C and 897D, these lithologies occur within four intervals, each of which constitutes a crudely upward-fining sequence, passing up into the silty claystones of Subunit IIIA.
The four intervals are described below using the facies types defined by Pickering et al. (1986). From top (4) to bottom (1), the sequence is as follows:
4. Highly variegated interval of thin beds of sandy siltstone to sandy claystone within silty claystone and coarse clayey/silty sandstone (Intervals 149-897C-62R-1, 30 cm, to -62R-2, 150 cm, and -6R-1, 0-94 cm; Fig. 15). Sandy beds in this interval exhibit features similar to Facies B1.2 (thin-bedded, coarse-grained sands).
3. Thin-bedded, upward-fining sequence of granule conglomerate and very coarse sandstone that are poorly sorted, matrix-rich, polymictic, and poorly cemented (Intervals 149-897C-62R-3, 0 cm, to -62R-2, 9 cm [Fig. 16B–16C] and 149-897D-6R-1, 90-110 cm). Within the poorly sorted conglomerate, the sand-silt-clay matrix is similar in composition to the finer-grained sediments within this subunit. This interval exhibits features similar to Facies A2.3 (normally graded gravel).
2. Very poorly sorted, matrix-rich conglomerate (Intervals 149-897C-62R-3, 40 cm, to -62R-4, 50 cm, and 149-897D-6R-1, 0 cm, to -6R-3, 101 cm [Fig. 16A]). Granule- and pebble-sized clasts within the conglomerate framework range from angular to subrounded; maximum clast size is 4 cm. The granule-conglomerate is structureless, except for a poorly defined imbricate alignment of clasts with bedding. This interval exhibits features similar to Facies A1.1 (disorganized gravel).
1. Interval dominated by clasts of yellowish nannofossil clay-stones and chalks ranging between 2 to 7 cm in diameter, but also including a basalt clast at Interval 149-897D-6R-3, 46 cm, and a fine-grained limestone granule at 149-897C-62R-4, 42-43 cm (Fig. 17). Nannofossil evidence indicates that the claystone/chalk clasts are of different ages (late Hauterivian, Barremian, early Aptian) (see "Biostratigraphy" section, this chapter).
Color in subunit IIIB is extremely variegated and includes moderate reddish to dark reddish brown (10R 4/6; 10R 3/4) and shades of gray, green, and blue (5GY 5/1, 5G 5/1, 5YR 4/1, 5G 6/1, 5B 6/1). Color variations are partly the result of diagenetic effects (e.g., oxidation/reduction) and in part the result of variable clast composition. Parallel lamination, defined by color-banding, characterizes the fine-to-coarse sandstones within the claystones in the upper part of the subunit.
Type 2 claystones are the dominant lithology of Subunit IIIA, with a minor silt admixture that includes quartz, feldspars, and MRFs. Coarse-grained sandstones of Unit III are litharenites (classification of Folk, 1980). Many lithic clasts contain, or are coated with, fine-grained and opaque iron oxides (red-brown in reflected light) that obscure their identity. Other opaque grains are white in reflected light, which suggest that fine-grained Ti-oxide is also present as a grain alteration product. Identified lithic types include terrigenous clastic grains (claystones, silty claystones, clayey siltstones, and sandstones), limestones, clayey limestones, possible dolomites, and mafic volcanic grains that have been altered to widely varying degrees (red-brown to green). Black opaque grains are abundant. Some of the carbonate lithologies contain shallow-water allochems.
Several highly lithified pieces of sandstone recovered in Unit IV have been interpreted as caved material from the overlying Unit III and thus are described here. Two sandstone samples were examined in thin section.
A carbonate-cemented moderate brown (5YR 4/4) coarse sandstone from Section 149-897C-64R-1 at 7 cm is an endmember litharenite (classification of Folk, 1980). Rock fragments make up approximately 75% of the volume of total rock and consist of carbonate rock fragments and serpentine clasts. The serpentine fragments are oxidized to varying degrees and vary in color from mottled yellowish brown to green. A few clasts of claystone and clayey siltstone also are present. The carbonate cement is crudely fibrous in part and in part made of equant spar. A portion of the detrital carbonate consists of monocrystals and, hence, the identifying grains from cement is ambiguous in many places.
A sandstone (greenish gray, 5GY 6/1) from Section 149-897C-64R-1 at 34 cm is a medium-sized carbonate-cemented lithic arkose with 40% of mostly monocrystalline quartz. Feldspar (20% of the grains) consists mostly of K-feldspar, including microcline. Minor Ca-plagioclase grains are highly vacuolized and otherwise altered. Rock fragments (40% of the grains) are mostly CRFs. Carbonate lithologies and allochems include micrite, fragments of benthic foraminifers, brachiopods, and red algae. Minor rock fragments include mica schist, basalt, serpentine, and mica. Several biotite grains have expanded fabrics, apparently produced during growth of the authigenic carbonate.
Significant numbers of possibly altered silicate grains are present that have been replaced by one or more authigenic minerals having low, first-order birefringence. Some apparent replacements are micro-crystalline with low relief. Another phase has moderate relief and prominent cleavage. Possible minerals corresponding to these phases include various zeolites and clay minerals.
Marly limestone and sandstone pebbles or clasts may be representative of the rocks present in Lower Cretaceous units along the west Iberia margin (Boillot, Winterer, Meyer, et al., 1988; Sibuet, Ryan, et al., 1979). Altered basaltic clasts (see "Igneous and Metamorphic Petrology and Geochemistry" sections, this chapter) are present in the gravel and coarse sandstones of Subunit IIIB. Sand-sized basaltic fragments occur in the thin clayey sandstones in the interval from the top of Subunit IIIB to Interval 149-897C-62R-3, 0-150 cm. These thin sands may represent highly altered ash layers and are evidence of possible contemporaneous volcanic activity.
Cursory examination of the clay mineral assemblage in Unit III reveals the presence of illite, kaolinite, an expandable clay, and possible palygorskite.
Subunit IIIA is interpreted as the product of slow accumulation of clay in an oxygenated environment, possibly beneath the CCD. The clay and fine silt are most likely continental material supplied by low-density turbidity flows or by the nepheloid layer of contour currents. Comparison with other DSDP/ODP sites along the north-eastern Atlantic continental margin (Laughton, Berggren, et al., 1972; Sibuet, Ryan, et al., 1979; Boillot, Winterer, Meyer, et al., 1988) suggests a probable Eocene-Paleocene age for this subunit, a period when the regional CCD was relatively shallow (Fig. 181, p. 375 in Emery and Uchupi, 1984).
Subunit IIIB is interpreted as being deposited from high-concentration sediment flows that were transported by high-density turbidity currents or as fluidized sand-silt-clay debris flows on relatively gentle slopes. The basal, poorly sorted, conglomerate/gravel interval can be considered as a deposit from a single debris flow. Rounded clasts of different lithologies, ages, and degrees of lithification suggest that the sediment derived from multiple preexisting units that cropped out at seafloor escarpments. Coeval tectonic activity may have affected slope stability, which triggered sediment flow.
Cores 149-897C-63R-l, 0 cm, to 149-897C-66R-1, 18 cm
Cores 149-897D-7R-l, 0 cm, to 149-897D-11R-1, 0 cm
Depth: 648.7-677.5 mbsf (Hole 897C) and 655.2-693.8 mbsf (Hole 897D)
Age: late Aptian to late Hauterivian(?)
Unit IV consists of intervals of serpentinized peridotite, ranging from 15 cm to 1.5 m thick, sandwiched between dark gray claystones and a variety of other lithologies including various breccias (Fig. 18). Sedimentary intervals in the unit yield ages that young upward and range from early late Hauterivian to early Aptian (see "Biostratigraphy" section, this chapter).
The top of Unit IV is taken at the top of the first cores beneath the nannofossil claystone/chalk conglomerates of Subunit IIIB.
The base of Unit IV is defined by the first occurrence of serpentinized peridotite uninterrupted by intervals of sedimentary rocks. In Hole 897D, a thick (4.5 m) claystone is present in Sections 149-897D-10R-3 and 10R-4; continuous serpentinized peridotite begins at Section 149-897D-11R-1, 0 cm. However, in Hole 897C, the location of the lower boundary is debatable because of the difficulty of distinguishing genuine sediment intervals from apparent sedimentary material that was introduced by drilling disturbance. The shipboard sedimentologists confidently identified in-situ sediments in Sections 149-897C-64R-1 and -64R-2 immediately above a thick interval of serpentinite that spans Sections 149-897C-64R-3 and -65R-1 and Interval 149-897C-65R-2, 0-98 cm; however, they debated whether the origins of the following four intervals were sedimentary or were caused by drilling disturbance:
The base of Unit IV has been placed immediately below the lowermost interval containing sedimentary rocks (4 above).
The proportion of sedimentary rock intervals recovered from Unit IV in the two holes is different. In Hole 897C, this is about 33% (i.e., about 6.2 m of basement in a total recovery of 9.2 m) and in Hole 897D, is about 85% (i.e., about 1.9 m of basement in a total recovery of 11.4 m).
Figure 19 provides a graphic summary of sedimentary lithologies and serpentinized peridotite recovered from Unit IV in the two holes. Table 4 shows the occurrence, color, grain size and texture, petrographic composition, and internal structures observed in each sedimentary lithology present in the unit.
Several pieces of highly lithified, carbonate-cemented, medium-to-coarse sandstones and granule conglomerates are present in the cores of Unit IV. Occurrences of these lithified materials are within the upper few centimeters of Cores 149-897C-63R to -66R, and Cores 149-897D-7R to -8R. As stated above, these lithologies bear no obvious compositional or textural similarities to other rocks that make up the bulk of Unit IV and may be highly lithified clasts that caved into the hole from the conglomeratic intervals of Unit III.
Tectonic structures within Unit IV are discussed in the "Structural Geology" section (this chapter).
When Unit IV was first encountered at Hole 897C, two hypotheses were considered for its origin by the Shipboard Scientific Party: (1) it represents a tectonic melange or megabreccia involving basement rocks and sedimentary cover, or (2) it originated as a mass flow deposit.
A tectonic origin has been discounted because, despite certain brittle deformation within the hard sedimentary rocks, it is unlikely that the soft sediments within Unit IV would have retained primary laminations and slump features had they been subjected to fault brecciation. Therefore, we prefer a mass flow origin for the sequence. Moreover, as the ages obtained from Unit IV indicate that it systematically youngs upward, we postulate that the emplacement of the mass flows occurred during late Hauterivian(?), early Barremian to late Aptian time. If emplacement had occurred later, it would have involved transport of peridotite blocks and Early Cretaceous sediments, which should have produced a disordered age pattern, with older intervals overlying younger ones.
The absence of graded deposits within the sediments of Unit IV suggests that the mass flow did not transport material over a great distance. Gravitational transport can occur without major mixing of seawater; thus, fluidized facies, either matrix-rich debris-flow or high-density turbidite deposits, will not necessarily exist. In extremely dense mass flows, mud acts as a mobile phase to permit downslope movement, whereas the rigid blocks of basement are a passive component of the flow. During gravitationally induced movement, the soft sediments suffer ductile deformation.
Indications are that some sediments in Unit IV were in a slightly lithified state during their transport by gravity sliding. For example, the laminated gray siltstone recovered in Interval 149-897D-7R-3, 10-24 cm (Fig. 20) exhibits microfaults that produced millimeter to centimeter offsets, suggesting that lithification had occurred prior to transport. Unlithified sand would probably have undergone complete mobilization.
Deposition of Unit IV possibly occurred at the foot of a fault escarpment that exposed serpentinized peridotite basement during the Early Cretaceous, but the ridge may have been uplifted after the deposition of Unit IV. It is likely that this unit is the same age as the "synrift" sediments in the inter-ridge location imaged in seismic reflection profiles (see "Site Summary" section, this chapter, Fig. 3).
The distinctive sedimentary association observed in Unit IV (i.e., highly varied sedimentary lithologies intermixed with, and overlying, highly calcitized and veined serpentinized peridotite) is not without precedent. For example, one similar succession was described by Tricart and Lemoine (1991) from the Queyras ophiolite of the western Alps. There, pelagic limestones and cherts rest unconformably on a basement-breccia pillow lava assemblage and have been succeeded conformably by pelagic limestones and shales containing ophiolite clasts and olistoliths of continental origin. No equivalents to the basalts and ophicalcite breccias were recovered at Site 897, although altered basaltic tuffs occur in Subunit IIIB and pieces of altered diabase are present in cores where Unit IV was recovered. More detailed comparison between comparable occurrences of ophiolite-associated sediments and Unit IV of Site 897 await post-cruise studies.