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doi:10.2204/iodp.proc.318.103.2011

Lithostratigraphy

One hole was drilled at Site U1355 to a total depth of 31.7 mbsf using the RCB system. Core 318-U1355A-1R recovered 5.30 m of sediment over a drilling advancement of 3.50 m. The core contained two fining-upward sand and gravel beds separated by a silty clay bed. Core 1R appears relatively undisturbed with preservation of sedimentary structures in fine sands, although portions of the gravel beds in this core were washed by the drilling process. The stratigraphic integrity of Cores 2R through 4R was highly compromised by drilling disturbance. Core 3R recovered 5 cm of sediment, all processed for micropaleontology. Drilling disturbance in Cores 2R and 4R is characterized by sands and coarse fall-in material.

Unit descriptions

Unit I

• Interval: 318-U1355A-1R-1, 0 cm, through 4R-CC, 22 cm

• Depth: 0.0–24.88 mbsf

• Age: Pleistocene to Holocene

Lithostratigraphic Unit I is composed of clast-supported moderately to well-sorted sandy granule-pebble gravels grading upward into well-sorted, fine, crudely stratified sands. A single 3 cm thick drilling-disturbed interbed of dark greenish gray diatom-bearing silty clay was preserved between two upward-fining units in Core 318-U1355A-1R (Fig. F3). In Cores 2R and 4R, the upper 40–100 cm of each core is characterized by disturbed coarse sands grading upward into fine sands. Occasional pebble- and cobble-sized clasts are present.

The gravel beds in Hole U1355A are polymict throughout. Clast lithologies are dominated by granitic gneiss, diorite, dark metamorphic rocks, and metasediments. Olive-gray intraformational clasts composed of silt-rich diatom ooze with a late Pleistocene–Holocene diatom assemblage, diatom clayey silt, and silty clay are also present. Rock clasts are angular to subangular in shape, and some pebble-sized clasts are faceted.

The silt and sand material recovered from Hole U1355A, as estimated from smear slides, is composed of mixtures of generally <50% quartz, with accessory feldspars, heavy minerals, and lithics. The silty intraclasts consist of 10% to >50% diatoms, with subordinate radiolarians and sponge spicules. The dark greenish gray diatom-bearing silty clay bed in interval 318-U1355A-1R-3, 91–94 cm, at the top of one of the fining-upward beds, has ~10% diatom fragments and other highly fragmented biogenic siliceous debris.

Discussion

Based on the preservation of three relatively intact sections in Core 318-U1355A-1R, we infer that the likely mechanism for the formation of the fining-upward beds is gravity flow. The texturally and compositionally immature character of the sediments and the angular clast shapes indicate that traction transport was minimal. The fining-upward texture of the beds indicates fluidal flow, with sufficient water sheared into the flow to allow particle size segregation by gravity settling. We interpret this as a high-density turbidity current (Lowe, 1982). The intraformational clasts of late Pleistocene–Holocene diatom ooze and silty clay could be interpreted as rip-up clasts, but it cannot be ruled out that these result from fall-in derived from material from the modern seafloor. The age of the flow is late Pleistocene or younger based on the diatom assemblages within the diatom-bearing silty clay (interval 318-U1355A-1R-3, 91–94 cm) in the portion of Core 1R that was only slightly disturbed.

The terrigenous material within the sediment has a clear glacigenic signature. Therefore, the source material for the flow must have been a glacigenic sediment. Glacigenic sediments were likely transported as debris flows with a transition to high-density turbidity currents through shearing at the debris/water interface and entrainment of seawater. Gravity flow could have commenced at any point along the outer continental shelf, continental slope, or continental rise. Both local winnowing of the matrix of older Neogene debris flows or winnowing of contemporaneous debris flows derived from higher up the continental slope is possible (Escutia et al., 2007).

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