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doi:10.2204/iodp.proc.318.109.2011

Lithostratigraphy

Two holes were drilled at Site U1361. Hole U1361A reached a total depth of 388.0 meters below seafloor (mbsf), using the APC system to refusal at 151.5 mbsf, followed by XCB drilling to the bottom of the hole. Hole U1361B reached 12.1 mbsf using the APC. Five lithofacies (Facies A–E) were identified at Site U1361 and, based on their distribution in Hole U1361A, two lithostratigraphic units are defined (Fig. F3). Facies A and B consist of clays and silty clays with common diatoms and foraminifers and rare decimeter-scale sets of millimeter- to centimeter-scale silt and clay laminations. These facies are restricted to the interval between 0.0 and 34.9 mbsf (lithostratigraphic Unit I). Facies A and B were deposited in hemipelagic depositional environments, with isolated sets of silt and clay laminations indicating occasional sedimentation from low-density turbidity currents or saline density flows consistent with a distal levee setting (Escutia et al., 2008). Facies C and D are strongly bioturbated silty clays and diatom/nannofossil oozes with intervals containing dispersed clasts. Facies E consists of laminated clays. Facies C–E are present between 34.9 and 388.0 mbsf (lithostratigraphic Unit II) and are typical of contourite facies associations (Stow and Piper, 1984), although downslope currents possibly contributed sediment as well. A decrease in diatom abundance was noted in smear slide observations below ~313 mbsf (Fig. F5; see also Site U1361 smear slides in “Core descriptions”). Pyrite is present in trace amounts throughout the hole.

Facies descriptions

Five lithofacies were identified in Holes U1361A and U1361B and are designated with capital letters (A–E) to avoid confusion with the facies reported at Site U1359 (see “Lithostratigraphy” in the “Site U1359” chapter):

• Facies A is characterized by light yellowish brown massive diatom-rich clay (Fig. F6).

• Facies B consists of light greenish gray to olive-brown silty clay with common foraminifers and fish fragments. This facies is mostly massive in structure (Fig. F7), although rare packages as thick as ~35 cm of millimeter- to centimeter-scale silt and clay laminations occur locally (Fig. F8).

• Facies C is characterized by light gray to white clay-rich nannofossil ooze and nannofossil-rich clay (Fig. F9). This facies is strongly bioturbated in most intervals and locally appears to be structureless.

• Facies D is characterized by light greenish gray to light gray bioturbated diatom-bearing to diatom-rich silty clay and clay-rich diatom ooze (Fig. F9). This facies commonly has dispersed gravel-sized clasts and is moderately to strongly bioturbated. Burrows as wide as ~2 cm are present throughout and are commonly pyritized.

• Facies E consists of olive-gray laminated clay (Fig. F9). This facies has common parallel laminations of silt and fine sand or displays a horizontal planar fissility where lithified. Bioturbation is sparse and consists of millimeter-scale (as large as 0.5 cm) burrows. Gravel-sized clasts are rare.

Unit descriptions

Based on the distribution of lithofacies, the sedimentary succession recovered at Site U1361 is divided into two lithostratigraphic units (Fig. F3). Unit II is further distinguished from Unit I by a distinct change in color, degree of bioturbation, and lamination frequency.

Unit I

• Interval: 318-U1361A-1H-1, 0 cm, through 4H-5, 88 cm

• Depth: 0–34.9 mbsf

• Age: Pleistocene

Unit I is composed of interbedded and structureless light yellowish brown diatom-rich clays (Facies A) and light greenish gray silty clays (Facies B). The silty clays contain common foraminifers and fish fragments. Nannofossils occur in a silty clay interval with foraminifers in a smear slide from Sample 318-U1361A-3H-5, 147 cm. Sand and silt interbeds (as thick as 8 cm) and rare packages of silt and clay laminations (as thick as ~35 cm) are present. Concentrations of granules and pebble-sized igneous and metamorphic clasts were observed as well.

Interpretation

The mixed terrigenous and biogenic components in Unit I indicate hemipelagic deposition. The sets of silt and clay laminations were probably deposited by low-density turbidity currents or saline density flows with traction transport, although a contour current origin is possible as well. The sand and silt interbeds could be derived from turbidity currents or sandy debris flows, but they may also represent lag deposits formed by current scouring. The presence of foraminifers and nannofossils suggests the site maintained a position above the calcite compensation depth, at least during some time intervals. Ice rafting is indicated by the dispersed gravel.

Unit II

Subunit IIa

• Interval: 318-U1361A-4H-5, 88 cm, through 34X-2, 14 cm

• Depth: 34.9–313.2 mbsf

• Age: late Miocene–Pliocene

Subunit IIb

• Interval: 318-U1361A-34X-2, 14 cm, to 41X-6, 14 cm

• Depth: 313.2–388.0 mbsf

• Age: late Miocene

Unit II is characterized by meter-scale alternation of laminated olive-gray silty clay (Facies E) and bioturbated greenish gray diatom-rich silty clay (Facies D) with minor diatom and nannofossil ooze and nannofossil-rich/nannofossil-bearing silty clays (Facies C). Some calcareous beds in the lower part of the unit are micritic and contain only trace amounts of preserved calcareous nannofossils. The olive-gray silty clays are characterized by parallel silt laminations or horizontal planar fissility. Bioturbation is sparse and mainly composed of millimeter-scale burrows. The greenish gray diatom-rich silty clays are commonly bioturbated with burrows as wide as 2 cm. The nannofossil-bearing lithologies are strongly bioturbated. Clasts are more common in the diatom- and nannofossil-rich facies than in the olive-gray silty clays. Conspicuous green laminations, with as large as sand-sized green minerals (possibly glauconite) are observed below ~155 mbsf (Fig. F9).

Subunit IIa is distinguished from Subunit IIb by the occurrence of nannofossil-bearing and micritic lithologies (Facies C) in Subunit IIb (Fig. F3). The interbedded bioturbated light greenish gray silty clays (Facies D) and laminated olive-gray claystones (Facies E) within Subunit IIb are similar to those observed in Subunit IIa. However, in Subunit IIb the light greenish gray bioturbated facies generally have a lower diatom abundance (diatom-bearing silty clays in Subunit IIb versus diatom-rich silty clays in Subunit IIa). Within Subunit IIb, Facies C–E are arranged in decimeter- to meter-scale cycles with sparsely bioturbated, sharp-based, planar-bedded olive-gray claystones at the base grading upward into moderately bioturbated, light greenish gray diatom-bearing claystones with light gray to white strongly bioturbated nannofossil-bearing clays at the top (Fig. F9).

Interpretation

The cyclic nature of sedimentation of Facies C–E likely represents alternating deposition from contour currents or downslope currents with minor traction (e.g., saline density flows or turbid plumes) and hemipelagic sedimentation. Differences in bioturbation intensity of the facies may result from changes in bottom water oxygenation. The dispersed gravel in Facies C and D indicates ice rafting. The higher concentrations of gravel in the nannofossil- and diatom-bearing facies (Facies C and D), as compared to the claystones (Facies E), could result from lower background sedimentation rates of fine-grained terrigenous material. Alternatively, greater calving rates or melt rates of icebergs in association with higher sea-surface temperatures coincident with hemipelagic sedimentation could have produced the larger gravel component.

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