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doi:10.2204/iodp.proc.334.105.2012

Lithostratigraphy and petrology

Site U1380 was drilled to investigate the lithostratigraphy of the lower portions of the upper slope sequence and the uppermost portions of the basement imaged in multichannel seismic reflection data. Coring in a single hole from 397 to 474.75 mbsf recovered sediment and sedimentary rocks. The hole was terminated above the contact with the basement (as inferred from seismic imaging) because of deteriorating hole conditions. The monotonous sequence of silty clay recovered at Site U1380 is punctuated by widely interspersed centimeter-scale sandy layers and four tephra horizons.

Description of unit

The 77.75 m sequence (Cores 334-U1380A-2R through 10R) recovered at Site U1380 is assigned to a single lithostratigraphic unit (Fig. F2).

Unit I

• Interval: 334-U1380A-2R-1 through 10R

• Thickness: 77.75 m

• Depth: 397–474.75 mbsf

• Age: late? Pliocene to late Pleistocene

• Lithology: clay(stone), silt(stone), and sand(stone)

Unit I is an olive-green clayey silt(stone) and silty clay(stone) with minor layers of sand(stone), sandy silty clay(stone), clay, clayey silt(stone), and tephra (Fig. F3). The sediment is firm and well consolidated and contains tephra layers that remain unlithified to 449 mbsf (Section 334-U1380A-7R-5). In general, Unit I is massive with minor changes in the proportions of clay, silt, and sand grain sizes occurring gradually over decimeter-scale intervals. Sandy intervals become thicker and more common with depth.

Unit I consists of fining- and coarsening-upward decimeter-scale sequences of sands. In the coarser sand layers, rip-up clasts, rounded clay lenses, and abundant shell fragments are commonly observed. Framboidal pyrite was observed in many of the smear slides throughout Unit I. Shell fragments, diatoms, and nannofossils are sparse but ubiquitous; foraminifers are partly enriched within the sediments and are a major constituent of the sand-sized fraction (especially the sandy silt[stones]).

Unit I is predominantly a terrestrially sourced sequence that is dominated by clay-sized particles. Smear slides indicate that the most common accessory grains in the silt(stone) and clay(stone) include feldspar and lithic (sedimentary and magmatic) fragments. Components present in trace abundances include amphibole, calcite, biotite, chlorite, glass shards, and opaque minerals. The main components in the sandstones are lithic (sediment > magmatic) clasts and feldspar crystals. Biogenic components are rare but include nannofossils, diatoms, and foraminifers.

Tephra layers

Hole U1380A cores contain four tephra layers intercalated with background Unit I sediment. Individual tephra layers range in thickness from 0.5 to 35 cm. In general, tephra layers have a sharp but unconformable basal contact with the underlying terrigenous sediment but a gradual transitional contact with the overlying tephra-bearing pelagic sediment, and many are normally graded in grain size and well sorted (Fig. F4). All four of the tephra layers have inclined bedding with dip magnitudes ≥30° (see “Structural geology”). The top of one of the tephra layers (interval 334-U1380A-10R-2, 23–37 cm) is bioturbated. The tephra layers are predominantly composed of felsic, white, ash-sized particles (>90 vol%) containing mainly transparent colorless glass shards with angular blocky, cuspate, flat, and Y-shaped appearance.

With the exception of one tephra layer (interval 334-U1380A-5R-5, 29–34 cm), all other tephras show devitrification structures within glass shards and alteration. Grain size ranges from very fine to coarse ash (millimeter size). The mineral assemblages consist of plagioclase, pyroxene (hypersthene and augite), hornblende, and biotite.

Depositional environment and correlation to Sites U1378 and U1379

The cover sequence recovered from Hole U1380A is a terrestrially sourced upper slope sequence that is consistent with high sediment accumulation rates throughout the depositional interval (see also “Paleontology and biostratigraphy”). The clay-rich deposits of Unit I in Hole U1380A are similar to those observed in Unit II in Holes U1378B and U1379C, although tephra chronologies will be required to correlate among the three sites.

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