LITHOSTRATIGRAPHY

At Site 1202, four holes were drilled: 1202A, 1202B, 1202C, and 1202D. Time constraints limited shipboard sedimentological core processing to Hole 1202A, upon which visual core description and color photospectroscopy were performed. The remaining cores were processed and described at the ODP Gulf Coast Core Repository at Texas A&M University ~4 months postcruise, including smear slide analysis of sediment samples from Hole 1202D. Recovery was very good in three holes: 1202A (106.4%), 1202B (102%), and 1202C (105.2%) and good in Hole 1202D (78.4%). The sedimentary succession consists of one lithostratigraphic unit, which is characterized by homogeneous, bioturbated clayey silt with isolated sandy intervals.

Depth: 0-120.07; 0-139.33; 0-97.78; and 0-408.10 mbsf

Age: late Quaternary (<0.46 Ma)

Unit I extends through the entire recovered succession in Holes 1202A, 1202B, 1202C, and 1202D. The color of Unit I from Hole 1202A (described on board ship) was dark greenish gray. For the cores described at Texas A&M University, Unit I is dark gray and becomes predominantly olive downcore. The color difference between the shipboard and repository descriptions is likely attributable to oxidation of the cores during their transit to Texas A&M University and storage as whole cores for 4 months before splitting. Dark gray horizons (centimeter scale) first occur in Sections 195-1202A-8H-1, 195-1202B-5H-1, 195-1202C-4H-1, and 195-1202D-8H-1 and are common throughout the remainder of the core (Fig. F3). Dark gray mottling occurs throughout the cores but is concentrated in some intervals. Small (millimeter scale) black and brown mottles (burrows?) partly comprised of sand-sized material are common in the uppermost cores and streak vertically through the core during cutting. The entire section is slightly disrupted by horizontal gas voids.

Unit I is characterized by homogenous, slightly calcareous, bioturbated clayey silt with isolated sandy intervals and fine sand laminae. Macrofossils and shell fragments are rare but are visible in most sections. Some sandy intervals (<5 cm thick) are normally graded with sharp, erosive basal contacts (e.g., Sample 195-1202D-17X-5, 132-135 cm) (Fig. F4). There are rare occurrences of inversely graded fine sands (e.g., Sample 195-1202D-31X-6, 24-35 cm), laminae within sand intervals (e.g., Sample 195-1202D-16X-6, 9-11 cm), and medium to coarse sands that are not calcareous (e.g., Sample 195-1202D-32X-4, 66-69 cm). More commonly, however, sandy intervals react strongly with 10% HCl and have been identified as detrital carbonate in smear slides. Sandy laminae also occur as thin (<5 mm thick) discontinuous stringers, thin beds (<5 cm), or burrows (Fig. F5). Sandy intervals increase in abundance and thickness at Core 195-1202D-20X. At this point, the recovery in Hole 1202D, the only hole that penetrated to this depth, begins to decrease. This is likely due to the washing of unconsolidated sands downhole during drilling and suggests that thicker sandy units prevail throughout the interval of poor recovery. Downhole from Core 195-1202D-31X, the abundance and appearance of the sandy units resemble those described above the interval of poor recovery.

Smear Slide Analyses

Microscopic smear slide analyses were carried out on sediment samples from Hole 1202D and revealed low downcore variability in textural and compositional features (see "Site 1202 Smear Slides"). The major lithology consists of clayey silts, mostly with <25% clay and high abundances of medium to coarse silt. Intercalated sandy intervals comprise sandy silts and sands with proportions of fine-grained and medium sand between 25% and 70%.

In all lithologies, the nonbiogenic sediment fraction is dominated by quartz and feldspar grains and abundant detrital carbonate clasts. Clay minerals form part of the fine silt and clay fractions. Some of the larger sand grains represent lithoclasts derived from marls and calcareous mica schists. The sand and coarse silt fraction includes mica, heavy minerals, and opaque minerals in minor amounts (mostly <5%). Indentified heavy minerals are green hornblende, epidote, tourmaline, and zircon, but others might also be present. In Sample 195-1202D-17X-3, 113 cm, one grain of blue hornblende was recognized. Volcanic glass shards are rare and were only observed in Sample 195-1202D-14X-6, 120 cm.

Biogenic components are present in minor amounts (<20%). They consist of biosiliceous material, mostly diatom and radiolarian remains, and calcareous nannofossils. Foraminifers are rarely seen in smear slides. Biosiliceous remains tend to decrease downhole, with maximum amounts above 65 mbsf and rare amounts below 220 mbsf.

The sedimentological analysis at Site 1202 enables a few constraints to be identified regarding the depositional environment at this locality within the Okinawa Trough. The site is characterized by an extremely high sedimentation rate, possibly in excess of 3 m/k.y. (see "Biostratigraphy"). This rate cannot be attributed to biological activity alone, as suggested by the low biogenic content, and implies a large terrigenous sediment input from an external source, likely reworked material from the shelf of the East China Sea supplied by turbidity currents through canyons to the Okinawa Trough. Clayey silts likely have been deposited by permanent contour currents, whereas graded sand layers indicate episodes of turbidity current activity. Thicker sandy intervals and relatively poor recovery beginning at Core 195-1202D-20X and continuing through 31X suggest that larger turbidity current events characterize this interval. Sources may include mountain ranges on the island of Taiwan, as suggested by detrital carbonate and high mica content, possibly from low-grade metamorphic schists and marls found within these ranges. The oceanic setting at Site 1202 north of the Ilan Sill, together with the high sedimentation rate, promotes nutrient upwelling and preservation of organic matter content as indicated by the dark color of the sediments and the smell of H₂S when opening the cores. No discrete ash layers were identified; however, preliminary smear slide analysis identified one sample with volcanic glass particles, indicating that the sediments likely include a small proportion of admixed ash particles. Discontinuous sand laminae, sand-filled burrows, and the homogeneous nature of the sediments suggest that bioturbation has reworked the core material extensively. Shallow-water foraminifers and wood fragments were identified within the coarse-grained component (see "Biostratigraphy").