LITHOSTRATIGRAPHY

The 136-m-thick hemipelagic sediment sequence probably spanning the last ~170 k.y. was recovered at Site 1233. A single lithologic unit (Unit I) was defined primarily on the basis of visual core description and smear slide examination and is dominated by clay and silty clay with varying amounts of calcareous nannofossils. Siliciclastic components include abundant clay minerals and feldspars and lesser amounts of quartz. Amphiboles, pyroxenes, mica, and opaque minerals are present as minor components throughout Unit I. Biogenic constituents vary in abundance and consist primarily of nannofossils with less abundant diatoms, spicules, silicoflagellates, and foraminifers. The sediments contain authigenic pyrite and monosulfidic minerals appearing as discrete dark spots or as finely dispersed particles, causing a distinct color mottling in the otherwise homogeneous olive-gray to dark olive-gray sediments. Rare interbedded minor lithologies include thin silt and silty sand layers and volcanic ash layers.

Sediment density generally increases with depth and reflects the effect of mechanical sediment compaction and dewatering. Magnetic susceptibility is lower in the uppermost part (~0-10 mcd) of the recovered sequence, increases downhole to ~20 mcd, and shows two low-intensity intervals (~93-98 and ~113-118 mcd) that correspond to higher chroma a* values (Fig. F7).

Except for the rare silt and silty sand layers that may represent distal turbidites, the lithology of sediments at Site 1233 is consistent with undisturbed hemipelagic sedimentation. Above-average sedimentation rates even for upper continental slope settings are likely the result of enhanced fluvial supply of terrigenous material induced by heavy precipitation in the mountainous continental hinterland.

Description of Lithologic Unit

Unit I

Intervals: Core 202-1233A-1H; Cores 202-1233B-1H through 11H; Cores 202-1233C-1H through 13H; Cores 202-1233D-1H through 13H; and Cores 202-1233E-1H through 8H
Depths: Hole 1233A: 2.50-7.53 mbsf (3.7-8.73 mcd); Hole 1233B: 0-109.59 mbsf (0.06-127.24 mcd); Hole 1233C: 0-116.77 mbsf (0-136.06 mcd); Hole 1233D: 0-112.78 mbsf (0-133.61 mcd); and Hole 1234E: 0-101.27 mbsf (0.34-117.85 mcd)
Age: late Quaternary (~0-170 ka) (see "Age Model and Mass Accumulation Rates").

Primarily based on visual core description and smear slide analysis, we distinguished one lithologic unit at Site 1233 (Unit I) (Fig. F7; Table T11). The dominant lithology defining Unit I is primarily homogeneous nannofossil clay, nannofossil silty clay, and nannofossil-bearing silty clay (Fig. F8). Some intervals contain more abundant siliceous microfossils (diatom nannofossil clay, diatom-bearing silty clay, and diatom-nannofossil-bearing clay) and less abundant clayey nannofossil ooze.

Sediments of Unit I display little visual variability and few sedimentary structures (Fig. F8). Thin fissures induced by degassing of the sediment are present throughout the unit. Sediment color ranges from dark olive-gray to dark gray with occasional meter- to decimeter-scale olive-gray to gray intervals. Color changes are subtle and gradational. Upon opening, most cores display abundant black spots and mottles along the surface (Fig. F9), originating from disseminated monosulfides that fade during the core processing time (1-1.5 hr). Bioturbation is rarely visible, probably because of the homogeneous lithology. Some cores contain randomly oriented isolated macroscopic shell fragments.

The siliciclastic fraction within the major lithology lacks sand. Silt contents vary from ~10% to 30% without any remarkable downhole trend (Fig. F10). Silt-sized siliciclastic components are dominated by feldspar (predominantly plagioclase) followed by quartz. Overall, the quartz/feldspar ratio decreases slightly downhole (Fig. F10). Amphiboles, pyroxenes, micas, and opaque minerals are present in variable minor amounts. Small amounts of volcanic glass, partly altered to palagonite, as well as authigenic monosulfides and pyrite are ubiquitous. Primarily, finely dispersed pyrite is present and occasionally is concentrated as framboids in biogenic material such as diatoms and foraminifers.

From ~0 to 40 mcd, biogenic components are fairly constant and are dominated by nannofossils and smaller amounts of foraminifers (Fig. F11). Downhole, the proportion of biogenic components is lower and more variable with an increased abundance of siliceous microfossils (predominantly diatoms, minor spicules, radiolarians, and silicoflagellates). Two biogenic-rich intervals appear between ~49 and 53 mcd and ~93 and 98 mcd, with nannofossil abundance exceeding 50%.

Calcium carbonate concentrations vary from ~2 to 10 wt% and roughly parallel the observed changes in the biogenic carbonate fraction (Fig. F11) (see "Geochemistry"). No smear slides were taken from the first carbonate maximum at ~9-19 mcd, but the lower part of the second interval of increased carbonate contents (~93-100 mcd) correlates with the occurrence of clayey nannofossil oozes and with a distinct minimum in the magnetic susceptibility record (Fig. F11). In contrast, the pronounced second interval of low magnetic susceptibility (~113-118 mcd) is nearly barren of calcareous and siliceous microfossils. Here abundant carbonate concretions suggest strong diagenesis (Fig. F11).

Silt-rich layers and volcanic ash layers are notable minor lithologies. Silt-rich (primarily clayey silt) layers (<1 cm) are present in two intervals from ~9 to 25 mcd and from ~104 to 113 mcd (Table T12). Two thicker (~7-20 cm) fining-upward silty sand layers were observed at ~89 and 93 mcd (Table T12). Volcanic ash layers consisting of up to 95% unaltered volcanic glass are present at ~15 mcd and in the 105- to 130-mcd interval (Fig. F12; Table T12). The five ash layers are of both acidic and intermediate composition (Table T12; Fig. F10). Acidic ash layers are characterized by andesine plagioclase and higher quartz and mica together with low pyroxene contents, whereas ash layers of intermediate composition contain labradorite plagioclase, less quartz, and mica, as well as more abundant pyroxenes.

Magnetic susceptibility is low in the uppermost part (0-10 mcd) of the recovered sequence, increases downhole to ~20 mcd, and fluctuates on a high level from 20 to 140 mcd with two prominent low-intensity intervals (~93-98 and ~113-118 mcd) (Fig. F7).

Color parameter a* indicates that sediments are generally more greenish (Fig. F7) in intervals with low magnetic susceptibility, with the exception of the first 20 mcd, where a* does not agree with magnetic susceptibility. This relationship suggests that reductive dissolution of oxyhydroxides is incomplete above 20 mcd, drawing a* toward red values (a* > 0). L* is constant at ~40%, with slightly lower values in the upper 20 mcd and higher values in some of the low-susceptibility intervals (Fig. F7). In the a*-b* color space, almost all color measurements at Site 1233 plot in the "yellow" domain (Fig. F13). Color parameter b* is weakly correlated with the total organic carbon (TOC) content (r2 = ~0.5). Assuming a two-component chromatic system, preliminary predictive relationships between reflectance, carbonate, and TOC (Fig. F14) were developed via a multiple linear regression with encouraging results (i.e., r2 = ~0.7 for carbonate, and r2 = ~0.8 for TOC).

GRA and moisture and density (MAD) bulk density increase with depth as a consequence of sediment compaction and dewatering, most evidently in the upper 20 mcd. MAD measurements also show this downhole trend (Figs. F7, F15). GRA density data and MAD discrete bulk density measurements show a very good correlation in the upper 7 m of the sequence. Below 7 mcd, however, an offset of ~0.1 to 0.3 g/cm3 between the two sets of density data degrades the correlation. The systematically lower values for GRA bulk density correspond to the lack of a P-wave signal below 10 mcd. Both effects are probably caused by gas expansion that generates voids and fissures within the sediments.

Interpretation and Depositional History

Sediments at Site 1233 are dominated by lithologically homogeneous fine-grained terrigenous material with varying amounts of well-preserved biogenic components (see "Biostratigraphy"). This lithology suggests that hemipelagic sedimentation predominates, with the exception of rare thin silt and silty sand layers that might represent distal turbidites. This is consistent with the physiographic setting of the site (see "Introduction"): a small forearc basin on the upper continental slope away from major pathways of turbidity currents, which are rather channelized in this region (Thornburg and Kulm, 1987). The preliminary age model (see "Age Model and Mass Accumulation Rates") suggests very high average sedimentation rates in the range of 100 to 170 cm/k.y. for the late glacial and Holocene sequence at Site 1233. Such sedimentation rates are above average, even for upper continental slope settings (approximately a decimeter per thousand years), and can be explained by extremely high terrigenous sediment supply (likely a result of enhanced fluvial discharge in response to heavy continental rainfall in mountainous southern Chile). However, preliminary paleomagnetic evidence suggests that sedimentation rates were lower by a factor of three prior to ~41 ka (~70 mcd) (see "Age Model and Mass Accumulation Rates"), which may indicate that changes in transport pathways of terrigenous material to Site 1233 and syndepositional sediment focusing within the small forearc basin could have changed through time. Mineral assemblages are consistent with a siliciclastic sediment provenance in both the Andes and the Coastal Range. These results are in agreement with clay mineral and major element composition data from gravity core GeoB 3313-1 from the same location (Lamy et al., 2001). The slight downhole decrease observed in the quartz/feldspar ratios at Site 1233 (Fig. F10) might indicate a relative increase in the supply of basaltic to andesitic Andean source rocks vs. predominantly metamorphic Coastal Range source rocks.

Magnetic susceptibility increases, whereas TOC and, to a lesser extent, calcium carbonate contents decrease from the Holocene (~0-12 mcd) to the late glacial, suggesting an enhanced terrigenous dilution of the biogenic component during the late glacial (see "Geochemistry"). This shift may be a result of increased late glacial continental rainfall as suggested for central Chile (~33) (Lamy et al., 1999) and/or may be induced by the glacial sea level lowering and resulting higher terrigenous supply to Site 1233.

In the interval from ~93 to 98 mcd, a maximum in calcareous components corresponds to both higher calcium carbonate contents and a pronounced minimum in magnetic susceptibility (Fig. F11). However, the second minimum in magnetic susceptibility from ~113 to 118 mcd is associated with very low calcareous microfossil abundance with poor preservation (see "Biostratigraphy"). Carbonate concretions and the dissolution of fine-grained ferromagnetic minerals (see "Paleomagnetism") indicate an early diagenetic overprint during this interval.

Five volcanic ash layers ranging from intermediate to acidic composition were observed at Site 1233. These layers may provide regional stratigraphic markers.

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