RESULTS

The textural data consist of a total of 172 samples described in detail: 47 samples from Site 897, 44 samples from Site 898, 47 samples from Site 899, and 34 samples from Site 900. These samples were selected during post-cruise studies on recovered sediments and were available for shipboard sampling based on their relative abundance in the cores and as representative of all the sedimentary facies described on the core barrel sheets on board JOIDES Resolution.

The results of the textural analysis of selected samples of lithostratigraphic Units I and II from Sites 897, 898, 899, and 900 are shown in Table 1. The binary plots in Figure 2 and Figure 3 show the relationship of standard deviation vs. mean particle size and kurtosis vs. skewness. On these plots, all samples analyzed from lithostratigraphic Units I and II are represented. The diagrams are indicators of the hydraulic regime on the Iberia Abyssal Plain during the Cenozoic. According to Sly et al. (1983), these data indicate that Cenozoic sediments in the Iberia Abyssal Plain were deposited under low-energy conditions.

Sedimentological data such as texture, grain-size statistics, cumulative shape curves of the grain-size distribution, and sand fraction composition allow us to characterize the different Cenozoic sedimentary facies recovered on Leg 149 (Palanques and Maldonado, 1985; Comas and Maldonado, 1988; Alonso and Maldonado, 1990; Ercilla et al., 1994). Grain-size analysis of the sediments was oriented toward finding environment-diagnostic parameters/descriptors of the grain-size distribution (McLaren, 1981). Textural parameters for this study are interpreted as measuring different aspects of transport mechanisms, depositional environments, and the energy responsible for deposition (Blatt et al., 1980; Forrest and Clark, 1989).

According to the terminology of fine-grained sedimentary facies in deep water (Stow and Piper, 1984), three main sedimentary facies make up the Cenozoic section present in lithostratigraphic Units I and II at Sites 897, 898, 899, and 900: (1) pelagic and hemipelagic, (2) turbidite, and (3) contourite (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Plate 1). Our data confirm that during Cenozoic time, the main sedimentary processes in the Iberia Abyssal Plain consisted of a combination of differential pelagic settling, turbidity currents, and bottom currents, as stated by the Leg 149 Shipboard Scientific Party (1994).

Pelagic and Hemipelagic Facies

The pelagic and hemipelagic facies are present throughout all Cenozoic sections recovered at Sites 897, 898, 899, and 900. Pelagic sediments are defined as "deep-sea sediments without terrigenous material," and hemipelagic sediments are defined as "deep-sea sediments containing a small amount of terrigenous material as well as remains of pelagic organisms" (Bates and Jackson, 1987).Carbonate-rich pelagic facies are represented by calcareous biogenic sediments, composed predominantly of clay-size material (65%-82%). These pelagic facies are poorly sorted (1.32-1.93) muds with a bimodal distribution at 9 and a small peak at 3 (Fig. 4). The median grain size varies from 7.5 to 8.7, with a relatively low sand fraction percentage (3%-7%). Light-colored beds of clayey nannofossil ooze have a unimodal grain-size distribution at 8.5 and contain up to 80% clay-size grains. The cumulative curve of the fine-grained fraction is hyperbolic and logarithmic tending toward hyperbolic (Fig. 8). The carbonate content is normally high (65%-82%), except a few samples with a relatively low content (50%-58%). The sand fraction is composed mostly of well-preserved planktonic foraminifers.

Hemipelagic facies are characterized by a poorly sorted (1.63-2.77) mud with a bimodal distribution at 7.5–8 and a small peak at 3 (Fig. 4). The median grain size varies from 7.8 to 8.7 with a low (0.20%-6.40%) sand fraction content. The cumulative curve of the fine-grained fraction is hyperbolic and logarithmic tending toward hyperbolic (Fig. 8). The sand fraction is composed largely of well- and moderately preserved planktonic foraminifers, with a trace (<1%) of terrigenous components. The sand fraction of some samples (e.g., Samples 149-899A-15R-5, 54-57 cm, 149-899A-16R-2, 15-18 cm, 149-900A-5R-5, 35-37 cm, 149-900A-21R-4, 83-85 cm, and 149-900A-24R-2, 143-145 cm) is also formed by siliceous biogenic components (radiolarians, diatoms, and silicoflagellates). The carbonate content varies from <5% to 49%. The minimum values of the carbonate content correspond to samples formed by a high percentage of siliceous components.

Turbidite Facies

The turbidite facies include sediments with three different types of texture: sand, silt, and clay. We identified the Ta, Tb, Tc, Td, and Te divisions described for the Bouma (1962) turbidite sequences. The Ta and Tb divisions were recovered only at Sites 897 and 898. The Tc division was identified at Sites 897, 898, and 899. The Td and Te divisions were identified at all sites (897, 898, 899, and 900). The sand fraction of these sediments consists of terrigenous and terrigenous-biogenic deposits.

The Ta division shows two types of textures: coarse sand (<1.5) and fine sand (>1.5). The standard deviation (0.6) is better in coarse sands than in fine-grained sands (from 1.4 to 1.8 ). The size distributions are positively skewed. The coarse sands have a mean grain size at 1-1.5 and fine sands at 3 (Fig. 5). Settling curves for Ta fine-grained divisions are parabolic (Fig. 8). The carbonate content (6%-8%) is the lowest obtained for the turbidite facies. The sand fraction is composed mainly of terrigenous components (98%): mainly light minerals (70%), mica (3%), and pyrite (<3%). Some samples (e.g., 149-898A-5H-3, 59-61 cm, and 149-898A-10H-6, 95-97 cm) contain abundant mica (22%). Unidentified shell fragments are present in relatively significant percentages (40%) in some samples (e.g., 149-897A-1R-1, 25-27 cm).

The Tb division corresponds to a silty sand texture. On the basis of standard deviation, two different subtypes were identified: poorly sorted (1.50) and very poorly sorted (2.60) silty sands. The silty sands have a median grain size of 5, a skewness close to zero, and a mode at 3. The fine-grained settling curves have long tails toward the finest fractions (Fig. 5). The cumulative curve of the pelitic fraction is logarithmic with a parabolic tendency (Fig. 8). The carbonate content is low (8%-17%), and the sand fraction is composed of terrigenous components (88%-97%) that are mainly light minerals, pyrite, and mica.

The Tc division corresponds to very poorly sorted (2.80) silty sands and sandy silts with a median grain size of 5.0 to 6.3. These divisions have a mode at 3 with long tails toward the fine-grained fractions (Fig. 5). The cumulative curve of the pelitic fraction is logarithmic with a parabolic tendency (Fig. 8). The skewness varies from 0.2 to 1.0. The carbonate content is low (8%-24%). The sand fraction is composed of terrigenous (40%-100%) and biogenic components (0%-60%). Mica percentages are relatively high (31%) in some samples (e.g., 149-898A-12H-4, 32-34 cm).

The Td division corresponds to poorly sorted (1.6-2.0) silt and muddy silt textures characterized by a median grain size of 7.0 and by a skewness close to zero. These divisions have a mode at 5.5-6 (Fig. 5). The cumulative curve of the pelitic fraction is logarithmic tending toward parabolic (Fig. 8). The skewness values are positive close to zero. The carbonate content ranges from 6% to 40%. The sand fraction is composed of terrigenous (40%-98%) and biogenic components (2%-60%), mainly light minerals and planktonic foraminifers, respectively, and also includes traces (<5%) of mica, pyrite, and benthic foraminifers. Mica percentages are higher (58%) in some samples (e.g., 149-897A-1R-1, 63-65 cm).

Within the Te division, we identified the Te1, Te2, and Te3 divisions corresponding to Piper's (1978) sequence for fine-grained turbidite deposits. The Te division is composed of terrigenous (40%-99%) and biogenic components (1%-60%), with large amounts of light minerals and planktonic foraminifers. In general, mica, pyrite, and benthic foraminifers are present in trace amounts, but mica percentages are relatively high (57%) in some samples (e.g., 149-897C-49R-4, 13-15 cm).

The Te1 division is represented by poorly sorted (1.5 to 2.1) muds and is characterized by a median grain size of 7.7 with a positive skewness generally close to zero. These sediments have a mode at 7-8 (Fig. 6). The cumulative curve of the fine-grained fraction is logarithmic (Fig. 8). The total carbonate content varies from 4% to 42%.

The Te2 division is represented by poorly sorted (1.80) muds with a median grain size up to 8.0 and a mode at 7-8 (Fig. 6). The cumulative curve is logarithmic (Fig. 8). The carbonate content is variable, ranging from 3% to 33%.

The Te3 division is represented by poorly sorted (1.4-2.0) muds with high values of median grain size (8 to 9). These sediments have a mode at 9 , with long tails on the settling curve toward the fine-grained fractions (Fig. 6). The skewness is positive and negative close to zero. The cumulative curve of the fine-grained fraction is logarithmic (Fig. 8). The carbonate content ranges from 4% to 60%.

Contourite Facies

Separating turbidites from contourites has always been problematic, especially when bottom currents modify deposits containing turbidites (Faugères and Stow, 1993). In the Iberia Abyssal Plain, in addition to evidence for turbidity currents and pelagic settling, some evidence for occasional bottom-current activity exists in spite of the difficulty distinguishing lithologic features of modern and fossil contourites from turbidites.

This evidence is found in some sandy layers with a low pelite content (<19%-30%) that contain high concentrations (up to 90%) of coarse foraminiferal test remains. These layers were identified at Site 897 within Unit II (Sample 149-897C-30R-4, 40-42 cm), at Site 898 within Unit I, (Sample 149-898A-1H-5, 128-130 cm), and at Site 899 within Unit II (Sample 149-899A-14R-3, 62-64 cm). Rupke and Stanley (1974) found pteropod shell layers in the Algero-Balearic Basin; Comas and Maldonado (1988) found foraminiferal sands during Leg 103 in the Iberia Abyssal Plain; and Faugères et al. (1979) also described pure foraminiferal contourite sands in the North Atlantic Basin. Those authors have hypothesized that these deposits seem to be a winnowed concentration formed by normal bottom currents.

In addition to sandy sediments in the Iberia Abyssal Plain, silty sediments with mixtures of biogenous and terrigenous or only biogenous composition were recognized at all studied sites. These silty sediments were recognized in Hole 897C within Unit I (e.g., Samples 149-897C-10R-1, 88-90 cm, 16R-2, 37-39 cm, and 19R-3, 38-40 cm) and Subunit IIB (e.g., Sample 149-897C-40R-6, 71-74 cm); in Hole 898A within Subunit IIB (e.g., Samples 149-898A-19X-6, 84-86 cm, and 29X-3, 12-14 cm); in Hole 899A within Subunit I (e.g., Sample 149-899A-16X-2, 95-97 cm) and Subunit IIB (e.g., Samples 149-899A-14R-2, 108-110 cm, and 15R-5, 58-61 cm); and in Hole 900A (Samples 149-900A-21R-5, 28-30 cm, 25R-4, 15-17 cm, and 26R-3, 16-18 cm). These deposits could be attributed to the effect of bottom-current processes. The silty deposits are differentiated from the Td division turbidites on the basis of the following diagnostic features: (1) bimodal and polymodal grain-size distributions in the total cumulative curve, (2) the lack of vertical grading in terms of textures and sedimentary structures, and (3) poorly preserved nannoplankton.

Taking these data into account, we distinguished two types of contourite facies: (1) sandy contourites resulting from a combination reworking and deposition from bottom currents and (2) silty contourites resulting from the deposition action of bottom currents, according to Stow and Piper (1984).

The sandy contourites consist of poorly sorted (1.85-2.96) silty sands and have a median grain size of 5 with low percentages of the clay fraction (9%) (Fig. 6). The grain-size distribution is bimodal, with a main mode at 3-4 and a subordinate mode at 6-8 (Fig. 7). The cumulative curve of the pelitic fraction is logarithmic tending toward parabolic (Fig. 8). The carbonate content is uniformly high (41%-48%). The sand fraction is composed largely of planktonic foraminifers.

We discriminated two subtypes of silty contourites on the basis of sand composition and carbonate content: (1) carbonate silty contourites and (2) terrigenous silty contourites. The carbonate silty contourites are characterized by poorly sorted (2.0-2.2) clayey silts with relatively high amounts of the sand fraction (up to 26%). The main grain size varies between 6.0 and 6.9. The grain-size distribution is polymodal, with modes at 3.5-4, 5.5-6, and 7.5-8.5 (Fig. 7). The cumulative curve of the pelite fraction is parabolic tending toward logarithmic, but the curve shape is convex upward between 11 and 3, which is related to the presence of nannofossils. The carbonate content is relatively high (35%-47%). The sand fraction is formed by biogenic components (70%) represented mainly by planktonic foraminifers (66%) and siliceous biogenic clasts (15%-23%).

The terrigenous silty contourites consist of poorly sorted grains (2.0 -2.3) with a median grain size of 5.4 to 6.8. The grain-size distribution is bimodal, at 3.5-4 and 5-5.5, with a long tail in the finer fraction (Fig. 7). The cumulative curve of the pelitic fraction is parabolic and parabolic tending toward logarithmic (Fig. 8). The carbonate content is lower (6%-23%) than in the sandy contourites. The sand fraction is dominated by terrigenous components (80%), especially light minerals and mica.

Bulk mineralogy was determined for 28 samples chosen as representative of the sedimentary facies described for the Iberia Abyssal Plain. The relative percentages of the major crystalline components (illite, kaolinite, chlorite, quartz, calcite, dolomite, feldspar, and plagioclase) of sediments from Sites 897, 898, 899, and 900 are presented in Table 2. The major sedimentary facies, which were identified on basis of textural results, are also characterized by significant differences in the bulk mineralogy (Fig. 9).

The bulk mineralogy of the pelagic facies reveals a high calcite contents (53%-76%) with a low abundance of clay minerals (illite 11%-19%, kaolinite 0%-6%, chlorite 0%-8%) and quartz (8%-12%) and very low feldspar (0%-11%) and plagioclase (0%-4%) (Table 2, Fig. 9).

The bulk mineralogy of the hemipelagic facies shows variable quartz (15%-51 %) and calcite (10%-39%) contents, with a low clay-mineral content (illite 9%-23%, kaolinite 0%-8%, chlorite 0%-9%) and a low abundance of feldspar (9%-12%) and plagioclase (8%-21%) (Table 2, Fig. 9).

The turbidite facies exhibits a variable mineralogical composition. Three main types of turbidite deposits can be distinguished on the basis of their bulk mineralogical content: (1) turbidite sand and silt with a variable quartz content (32%-65%), which is inversely related to calcite content (4%-34%), low clay-mineral content (illite 6%-12%, chlorite and kaolinite 0%), and low amounts of feldspar (9%-12%) and plagioclase (11%-21%); (2) turbidite mud containing abundant amounts of clay minerals (illite 25%-36%, chlorite 23%-25%, kaolinite 11%-18%), with a relatively high quartz content (23%-25%) and very low to absent calcite content (0%-7%); and (3) turbidite mud that has higher calcite content (31%-46%) and lower clay-mineral (illite 14%-36%, chlorite 0%-17%, kaolinite 0%-13%) and quartz (15%-20%) contents than the type 2 turbidite facies (Table 2, Fig. 9).

The bulk mineralogy of the contourite facies shows relatively high contents of calcite (30%-47%), quartz (15%-42%), feldspar (10%), and plagioclase (10%-21%) and low contents of clay minerals (illite 6%-16%, chlorite 0%, kaolinite 0%). Sandy contourites have higher quartz and lower illite percentages than silty contourites (Table 2, Fig. 9).