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doi:10.2204/iodp.proc.336.106.2012

Lithostratigraphy

APC coring was used to sample the sediment overlying basement in Holes U1382B, U1383D, U1383E, and U1384A (Fig. F1). Thirty-three APC cores were recovered. When basement was encountered, one or two XCB cores were cut across the sediment/​basement interface. Sediment coring was conducted at the very end of the expedition, and only minimal core description was conducted.

The lithology of all four holes is dominated by foraminifer nannofossil ooze with variable proportions of foraminifers and nannofossils. Well-sorted foraminifers and occasionally fining-upward sand layers (turbidites) are present in all holes. An interval in Hole U1382B contains muddy gravels containing sand- to pebble-sized serpentinite, gabbro, and basalt clasts supported by a nannofossil ooze matrix.

Hole U1382B

In Hole U1382B, we cored 98.8 m and recovered 84.28 m (85% recovery). Ten APC cores and two XCB cores were obtained. Sediments are mostly light brown to reddish-yellow foraminifer nannofossil ooze. Some layers of foraminifer nannofossil ooze may contain sandy to pebbly angular black clasts. Three of these pebbly clasts are serpentinite and one is gabbro (Sections 336-U1382B-6H-8 and 7H-1). Some layers of muddy gravel have sandy to pebbly clasts. These muddy gravels are often supported by a matrix of nannofossil-rich ooze and are clearly the result of downslope transport. Cores exhibit weak to high coring disturbance (biscuiting, soupiness, and cracks). Two fining-upward sequences (Sections 336-U1382B-6H-5 and 10H-5) were identified. A typical fining-upward sequence occurs from the bottom to the top of an erosional base (e.g., a sandy facies settling on a muddy facies) and above a decrease in grain size in the successive layers made of the foraminifer nannofossil ooze. Although this sequence lacks of some typical parts of the Bouma sequence (Bouma, 1962), the fining-upward sequences overlying an erosional base appear to be turbidites.

At the base of the section, near the oceanic crust, foraminifer nannofossil ooze layers have a strong brown color (7.5YR 4/6 to 3/4)—perhaps the result of an enrichment in Fe and Mg coming from the basement. The basal section also contains fragments of basalt, gabbro, and serpentinized peridotite.

Hole U1384A

In Hole U1384A, we cored 96.2 m and recovered 94.1 m (98% recovery). Cores included 11 APC cores and 1 XCB core. Light brown to reddish foraminifer nannofossil ooze with variable proportions of foraminifers and nannofossils represents the dominant lithology recovered. Cores record slight soupy coring disturbance. As in Hole U1382B, the sediments near basement (Cores 336-U1384A-10H and 11H) are foraminifer nannofossil ooze with a strong brown color (7.5YR 4/6 to 3/4). The brownish tints might result from staining of nannofossils by iron oxides (Shipboard Scientific Party, 1979). The layers in turn differ in color because of varying ratios of stained nannofossils to foraminifers; the lightest layers are the foraminifer sands, which are typically 10–20 cm thick in this hole.

Sedimentary structures observed include parallel laminations, a slump, a lot of fining-upward sequences, and bioturbation. The slump observed contains clasts of darker, more consolidated nannofossil-rich ooze. Overall, 20 fining-upward sequences, interpreted as turbidites, were observed throughout the hole. In contrast to Hole U1382A, no rock clasts are present, and the sediments are not as affected by coring disturbance.

Site U1383

Two holes at Site U1383 were cored ~10 m apart: Holes U1383D and U1383E.

Hole U1383D

In Hole U1383D, we cored 44.3 m and recovered 48.65 m (110% recovery). Six APC cores and one XCB core were obtained. Light brown foraminifer nannofossil ooze with variable proportions of foraminifers and nannofossils represent the main lithology. Fining sequences and layers with sandy to pebbly black angular clasts were also observed. Core disturbance is minimal except in the last core (336-U1383D-6H), which is highly disturbed (soupy)—likely from flow-in resulting from a partial APC stroke. Because of this disturbance, it is not clear if the color change just above basement observed at other sites is present here. However, Core 336-U1383D-5H does exhibit some strong brown color.

Hole U1383E

In Hole U1383E, we cored 44.2 m and recovered 50.28 m (114% recovery). Six APC cores and one XCB core were obtained. Sediments recovered from Hole U1383E are represented by light brown to reddish-yellow foraminifer nannofossil ooze with variable proportions of foraminifers and nannofossils. Some layers containing well- to poorly sorted sandy to pebbly black angular clasts are present. Four fining-upward sequences were observed and interpreted as turbidites. No color changes in the basal sediments were observed, and core disturbance is moderate (cracks and soupiness).

Synthesis

The sediment sections observed during Expedition 336 are similar to those obtained during the previous drilling program (Shipboard Scientific Party, 1979, 1998; Timofeev et al., 1979). Turbidite sequences are present in all of the sections, but, unfortunately, no correlations can be made at this stage.

The color change in the basal section may be the result of Fe and Mn enrichment, but geochemical analysis must be done to confirm this hypothesis. Previous coring (Shipboard Scientific Party, 1979, 1998) identified a single lithologic unit (Unit I) composed of nannofossil ooze with varying amounts of foraminifers, clay, radiolarians and sand, and nannofossil clay. This unit was divided into Subunit IA, composed of nannofossil ooze containing varying amounts of clay, foraminifers, and radiolarians, and Subunit IB, composed of red clay with only ~20% carbonate. Foraminifer nannofossil ooze recovered from IODP Expedition 336 sites can be correlated with Subunit IA. Although we did not identify Subunit IB in the four holes cored, this subunit could correspond to the one or two cores above the sediment/​basement contact that have a dark brown color and are very homogeneous.

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