A sediment section extending to 362.1 mbsf (381.7 mcd) was recovered from three holes at Site 1232. The sediment record is dominated by turbidite deposits and is characterized by siliciclastic silty clay and clay (dominant lithology) and sandy silt (minor lithology). Clay minerals and feldspars are common, whereas amphiboles, pyroxenes, and quartz are present in minor amounts. Biogenic components are generally rare to absent. Sediment dry bulk density and GRA bulk density are variable, with mean values that do not shift markedly between the uppermost and lowermost sediments, except for slightly lower values in the uppermost 15 mbsf. Magnetic susceptibility is generally very high and variable, and sediment reflectance is low. All measured parameters display high variability on decimeter (0.2-0.5 m) scales. Color changes are subtle and gradational within the dominant lithology but contrast sharply with the minor lithologies.
A single lithologic Unit I was defined for the site on the basis of visual core description, smear slide examination, diffuse reflectance spectrophotometry, magnetic susceptibility, GRA bulk density, natural gamma emissions, and moisture and density (MAD) measurements (Table T6). The dominant lithology consists of massive light gray to gray silty clay and clay. Lithologic variability mainly consists of the interbedding of coarser layers within the silty clay and clay sediments. The coarser-grained layers are generally 1 to 10 cm thick and are present throughout the section at irregular intervals averaging ~30 cm. These layers contain a sharp basal contact and grade upward to gradational, poorly defined contacts with the dominant lithology. They are interpreted as the basal portion of turbidite sequences. The interbedding of coarse-grained layers with the silty clay exerts a predominant influence on the magnitude and length-scale of variability displayed by all of the measured parameters.
The dominant lithology defining Unit I displays little visual variability. It is primarily a massive unit with few sedimentary structures. Sediment color ranges from light greenish gray to light gray to medium gray, and most color transitions are gradual. Some mottling, occasional light brown nodules, and thin silt layers are present. Sand content is negligible within the dominant lithology, and silt content varies from ~10% to 40%.
A second important lithology is interbedded with the dominant silty clay and clay throughout Unit I. It consists of coarser sediment, typically silty sand or sandy silt, overlying a sharp basal contact and grading upward over several centimeters to silty clay (Fig. F7). The interbedding occurs on decimeter scales and displays strong contrasts in texture and most physical properties, although not in mineralogy.
A total of 1328 coarse layers were observed (Table T7), with 772 present in the 289 m of core recovered in Hole 1232A (371 m deep) (Fig. F8). This extrapolates to 991 layers over the drilled interval (2.7/m on average), although variability is detected on all depth scales. The coarse lower portions of the layers vary in thickness from <0.5 to 118 cm, measured from the basal contact to the first shift in texture and color above the darkest interval (Fig. F7). The combined length of these coarse intervals represents ~15% of the undisturbed sediment recovered by the APC at the site (Table T6). The basal contacts of the coarse layers are generally sharp, although rarely planar. Many of the contacts have centimeter-scale, possibly erosional, relief. Many of the contacts and layers are deformed and inclined from subparallel to nearly perpendicular to the overall bedding, perhaps due to drilling disturbance. Because the coarse layers grade upward into the silty clay of the overlying sediments, in most cases it is difficult to determine the exact top of the entire sedimentary packet. Some or all of the silty clay that is defined as the dominant lithology may be associated with the deposition of the coarse sediment. Some of the coarse layers present in the upper part of the stratigraphic section are soupy and disturbed.
The coarse, graded layers generally thin downcore. From 235 to 293 mcd (217.3-275.0 mbsf; Cores 202-1232A-24X through 29X), an interval of nearly complete recovery, no coarse-grained layers thicker than 4 cm are present, whereas the coarse layers shallower than 120 mcd have an average thickness >4 cm. Although this may reflect an environmental shift in the source areas of the turbidites, the deeper interval is also strongly influenced by coring disturbance.
Silt-sized components of the dominant silty clay lithology consist primarily of feldspars, with lesser amounts of amphibole, pyroxene, and quartz (Fig. F9). Minerals that are present in trace abundance include mica, rutile, garnet, zircon, brown volcanic glass, and palagonite. The mineralogy of the coarser interbedded layers is similar to that of the dominant silty clay lithology, with the exception of containing fewer clay minerals. Biogenic components are present in trace abundances, with a few exceptions (Fig. F9). Several thin layers contain abundant nannofossils, and one sediment pod contains abundant siliceous microfossils. Subtle shifts in mineralogy occur downcore. For example, opaque minerals are present throughout all cores from Site 1232 but generally reach minimum concentrations in the interval from 220 to 250 mcd (Fig. F9). Zircon and rutile are absent from this interval, although they are present in trace abundances shallower and deeper in the section. Similarly, titanite is absent above 250 mcd and is present in trace abundances downhole. These changes in mineralogy generally follow shifts in the thickness of interbedded coarse-grained layers.
Thin ash layers are present in interval 202-1232A-34X-3, 116-116.5 cm (336.61 mcd), and Section 202-1232B-3H-1, 110 cm (22.88 mcd).
Slight to moderate bioturbation is evident within some intervals, particularly in the cores recovered by APC. Bioturbation is particularly apparent above color transitions and is expressed as smoothly perforated contacts and sediment mottling. A highly bioturbated minor lithology consisting of light brown clayey calcareous ooze is entirely present as diffuse light brown mottling within medium gray clay in Section 202-1232A-13X-3.
Core logging data are highly variable superimposed on a more or less constant background, with the exception of the uppermost 22 mcd, which are characterized by lower magnetic susceptibility and NGR and lower than average GRA densities (Fig. F10). This high variability is the consequence of primarily textural and secondarily mineralogical changes (e.g., ferromagnetic mineral content that increases in coarser layers) between fine-grained and coarse layers. Density and porosity suggest a slight downhole trend resulting from sediment compaction and dewatering with increased overburden (Fig. F11). However, this trend is well within the superimposed variability generated by differences in lithology. The different characteristics of the upper 20 m are more evident in the MAD data than in the core logging data (Fig. F11) with high porosity, high water content, and, consequently, low bulk density. Several samples in this upper sequence also suggest grain densities that are higher than the downcore average. In general, a contrast in the large-scale texture and interbedding is present along with the switch to XCB coring at 112.5 mbsf in Hole 1232A (129.7 mcd) (Figs. F8, F9). This may indicate a bias in the recovery that favors collection of finer lithologies and thinner coarse-grained layers. Alternatively, the change to XCB coring may have been required by a true change in lithology, as clay-rich sediments often cause high pullout tensions during APC coring, although some of the shift to thinner coarse layers occurs below the shift to XCB coring. Drilling disturbance within the XCB cores increases downcore and may explain the muting of many measured parameters below ~120 mcd. Below this depth, changes in mineralogy are subtle and primarily influence components that are present only in trace abundances (Fig. F9). Therefore, no unit or subunit divisions are defined based on these changes.
In general, the GRA and MAD measurements are well correlated (r 2 = 0.997) (Fig. F12). Thus, the linear relationship between MAD bulk density and dry bulk density can be applied to the GRA density data in order to calculate predicted dry bulk densities, which are necessary for calculating MARs.
Magnetic susceptibility, GRA density, reflectance spectroscopy data (primarily b*, but also L*) vary in relation to observed textural changes. Coarse layers are characterized by high magnetic susceptibility and GRA density and low b* and L* values. In the a*-b* color space, almost all color measurements at Site 1232 plot in the "yellow" domain (~a* = 0) (Fig. F13) with silty sand layers plotting at the lower end (b* < 2) (Fig. F14). Smear slide estimates suggest that the presence of coarser grains (silt and sand) reduces the chroma (~b*) even in finer sediments.
The physiographic setting of Site 1232, ~200 km from a continental source and seaward of the trench, is consistent with the accumulation of hemipelagic sediments. For example, such sediments are present within the uppermost interval (202-1232C-1H-1, 0-8 cm) and contain ~15% biogenic components. However, the sediments of Unit I are predominantly siliciclastic, and the accumulation of 362 m of sediment in <0.8 m.y. suggests a sedimentation rate of >450 m/m.y., which is much higher than might be expected from typical hemipelagic sedimentation alone. The trace abundances of biogenic components within the dominant lithology are therefore mostly a result of dilution by terrigenous sediments. Analyses of surface sediments demonstrate that the trench system is not an effective barrier to sediments derived from the continent (Lamy et al., 1998). In addition, the interbedded coarse layers constitute a significant fraction of the stratigraphic column and contain negligible biogenic components. The sharp basal contacts, erosive scour relief, graded bedding, strong change in physical properties, and finer, gradational tops all indicate that these are turbidite sequences. Because some fine-grained overlying sediment may have been deposited along with the coarser basal portion of the interbedded layers as a turbidite sequence, the measured occurrence of coarse lithologies within 15% of Unit I provides only a minimum estimate of the influence of these turbidites at Site 1232.
The mineralogy of the sediments within the turbidites and the dominant silty clay is similar and is characterized by a very immature mineralogical assemblage (i.e., low quartz, high plagioclase, and significant amounts of amphiboles and pyroxenes), suggesting a predominantly andesitic to basaltic provenance. This provenance is compatible with the proximity and mineralogy of the Andes rather than a plutonic (i.e., Coastal Range) or a mafic oceanic source. This is consistent with the modern mineralogical composition off the central Chile region (Thornburg and Kulm, 1987b; Lamy et al., 1998). Compared to the Chile margin sediments farther north and south, both trench and continental slope surface sediments suggest the most immature mineralogical assemblages off central Chile are due to the overwhelming contribution of highly erodible source rocks from the Andes supplied by the high discharge of river systems in this region (Thornburg and Kulm, 1987b; Lamy et al., 1998). Sediments are transported downslope within large submarine fan systems in this region and are further distributed along the trench toward the north (Thornburg and Kulm, 1987a). Therefore, it is difficult to say how far or via what exact path the turbidites traveled to the site, although they are more likely to have come from the south, where the trench is filled and where the sediment source is largest. The erosive contacts and similar mineralogies of the interbedded dominant and secondary lithologies indicate the possibility that some portion of the vertical pelagic sedimentation has not been preserved at this site. The persistently similar mineralogy indicates little change in the character of the sediment source through the last few hundred thousand years. The 772 layers recovered from Site 1232 represent a minimum estimate that does not include layers that were eroded by subsequent events or were not recovered during drilling. Therefore, the average time step for turbidite deposition is most likely on the order of a few centuries within the 0.78-Ma maximum age of the section. Their recurrence may reflect paleoseismic activity or other processes that affect slope instability. The presence of thinner interbedded coarse-grained layers in the lower half of the section may reflect a more distant origin of the older turbidites or less intense events.