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doi:10.2204/iodp.proc.301.106.2005

Lithostratigraphy

Sediment coring

Holes U1301C and U1301D were discontinuously APC cored to 265.3 mbsf, recovering fine- to coarse-grained turbidites and hemipelagic clay. This is virtually the same sedimentary section that was cored in Holes 1026A and 1026C during Leg 168, 1–2 km to the north along the same buried basement ridge. Resampling much of the same sedimentary interval during Expedition 301 was justified because APC coring had not previously penetrated below ~100 mbsf in this area, and we wished to collect high-quality samples for microbiological and geochemical analyses, especially close to the sediment/basement interface and the underlying crustal aquifer.

Time constraints prevented continuous coring of the complete sedimentary section in Holes U1301C and U1301D, but much of the interval that was cored yielded excellent recovery and high-quality samples (Fig. F12). Exceptions to this rule included Cores 301-U1301C-5H, 13H, and 16H and Cores 301-U1301D-1H and 2H (recovery = 30%–40%), where coarse sand prevented complete penetration of the APC barrel. In addition, an aggressive whole-round sampling program (for microbiological and geochemical analyses) removed a substantial fraction of recovered core from the catwalk, preventing description or other analyses of these intervals.

Hole U1301C is located 100 m north-northeast of Hole U1301B (Fig. F1). Sediment thickness in Hole U1301B was 265.2 m, and Hole U1301C reached approximately the same depth (cored to 265.3 mbsf; recovered to 262.9 mbsf). As described later in this section, there is good reason to believe that coring penetrated essentially the complete sedimentary section in Hole U1301C.

Hole U1301D is located 20 m east of Hole U1301C (Fig. F1). Hole U1301D was cored only within the interval between 120 and 177 mbsf during the last 20 h of Expedition 301 operations. Because this coring was done at the end of the expedition and we had a short transit to port, sediments recovered from Hole U1301D were described rapidly and were subjected to minimal analysis on the ship.

Silt- and clay-rich cores from Site U1301 are of exceptionally high quality, even from depths below 250 mbsf, because we used the APC rather than the XCB or RCB. In contrast, cores recovered from sandy and gravely intervals are generally of poorer quality and often include intervals within which there was complete resuspension and settling of particles. Because of discontinuous coring, irregular recovery, and extensive whole-round sampling, we were unable to determine well-constrained lithologic boundaries for the primary stratigraphic units at Site U1301.

Based on the cores available for description, we defined two stratigraphic units from Hole U1301C: Unit I is a turbidite sequence, and Unit II is a hemipelagic clay sequence (Fig. F12). The true boundary between Units I and II occurs within the noncored interval between 197.1 and 235.8 mbsf; its approximate location may be inferred from its equivalent depth in nearby Hole U1026C (216.1 mbsf).

Stratigraphic units

Unit I

Intervals: Sections 301-U1301C-1H-1, 0 cm, through 2H-6, 50 cm, and 301-U1301-2H-6, 50 cm, through 16H-CC, 5 cm

Depth: 0–235.8 mbsf

Lithology: turbidite sequence

Unit I is composed of distal turbidite sequences and is ~220 m thick. It is divided into two Subunits (IA and IB) on the basis of changes in dominant lithology (Fig. F12; Table T4). Upward-fining sequences were observed throughout the recovered intervals of Unit I. We do not distinguish turbidites and debris flows as previously described for Hole 1026C (Shipboard Scientific Party, 1997) because (1) the division between dense turbidite and debris flows is arbitrary and (2) coarser-grained sections of the sequence were often disturbed during coring, preventing accurate assessment of grain size distributions.

Five lithologies were identified in Unit I: (1) dark greenish gray hemipelagic clay (Fig. F13), (2) finely laminated silt and clay with normal size grading (Fig. F14A), (3) fine- to medium-grained sands with normal size grading (Fig. F14B), (4) massive, thick, medium-grained sand beds (Fig. F15A), and (5) granule beds (Fig. F15B, F15C). Based on the presence of graded bedding and parallel laminations within the silt and sand beds, they are identified as turbidites, in which layers Ta and Tb of the Bouma sequence are identified. Unfortunately, it was not possible to recognize finer structures (e.g., cross-lamination and flute casts), perhaps in part because of coring disturbance. Smear slide observations indicate that the sand layers in Unit I are composed of quartz, plagioclase, green amphibole, pyroxene, magnetite, pyrite, and other opaque mineral grains (Figs. F16, F17).

The subunits defined at Site U1301 differ somewhat from those defined at Site 1026 during Leg 168. This results mainly from differences in coring techniques and recovery during the two expeditions. Coring was continuous during Leg 168, but the RCB was used below ~100 mbsf, resulting in underrepresentation of poorly consolidated, coarse-grained intervals. Discontinuous APC coring during Expedition 301 bypassed some depth intervals and recovered coarse-grained sediments from other intervals, but may also have resulted in an overrepresentation of sand and gravel due to "flow in."

Subunit IA (0–13.1 mbsf)

Subunit IA predominantly consists of thinly bedded hemipelagic clay layers interbedded with silty and fine-grained sandy turbidites (Fig. F13). The greenish gray sediment in the upper portion of Subunit I contains well-preserved diatoms and silicoflagellates (Fig. F16). No bioturbation was observed within the turbidites or hemipelagic clay.

Subunit IB (13.1–235.8 mbsf)

Subunit 1B is a coarse- to fine-grained sequence of thinly bedded to massive bedded turbidites and clay layers (Fig. F12). The top boundary for this subunit is located where clay is no longer the dominant lithology, and the bottom of this subunit is located at the first appearance of the hemipelagic clay of Unit II. There are several 5–6 m thick intervals within this subunit where clay is the dominant lithology, but silt, sandy silt, silty sand, and sand are generally more abundant. This subunit also contains several gravel layers.

The shallowest part of Subunit IB (13.1–56.7 mbsf) was recovered in Hole U1301C and contains thinly bedded clay, silty clay, sandy clay, silt, clayey silt, sandy silt, clayey sand, silty sand, and sandy interbeds, generally arranged in fining-upward sequences. There are a few intervals within which clay or sand are the dominant lithology, with individual layer thicknesses of tens of centimeters to ~1 m, but thinner layers having a mixture of grain sizes are more common. Many of the thin silt and clay laminations within this part of the section contain poorly preserved microfossils and rare wood fragments (Fig. F13).

The central part of Subunit IB (56.7–109.6 mbsf) was recovered from Hole U1301C and is composed of more massive sand layers with thin silty and clayey interlayers. There is a small interval composed mainly of clay (74.6–79.9 mbsf), but this interval includes sandy interbeds. The rest of this part of Subunit IB is composed mainly of 1–20 m thick graded sand-silt beds (Fig. F14). Thick sand intervals (>10 m) were recovered at 56.7–74.6 mbsf and 81.1–109.6 mbsf. The true thicknesses of many these sand beds is likely to be considerably less than what is represented in the recovered core because of "flow-in" during coring. Intervals containing soupy, unconsolidated sand having no internal structures are particularly suspect.

The depth interval 109.6–130.1 mbsf was poorly represented by cores recovered from Hole U1301C. Cores 301-U1301C-13H and 14H failed to penetrate significantly into hard layers, and the rest of the interval in this hole was drilled without coring. Core 301-U1301C-14H recovered material only in the core catcher, comprising well-sorted, subrounded granules, and the core catcher from Core 13H contained similar material. The granules include serpentinite, green amphibolite, quartzite, felsic volcanic, calcareous sandstone, and shallow-water shell fragments (Fig. F15). We initially considered defining a new subunit on the basis of this lithology, but subsequent coring in Hole U1301D demonstrated that the very coarse intervals recovered in Cores 301-U1301C-13H and 14H represented only a small fraction of the sedimentary section.

The depth interval 120.0–177.0 mbsf was cored in Hole U1301D, and we recovered primarily sandy and silty turbidites, clay, and occasional gravel layers. Thin fine-sand layers are commonly spaced at 20–50 cm intervals within clay beds. These sand, silt, and clay layers are compositionally similar to those found higher in the section.

The depth interval 178.1–197.1 mbsf was cored in Hole U1301C, once again recovering mainly thin bedded turbidites having a range of grain sizes. Section 301-U1301C-15H-5 has a 20 cm, light yellowish gray clay bed that contains microfossils (foraminifers and diatoms). It is the palest lithology recovered from Hole U1301C. This clay bed was deposited during a time at which there was little continent-derived sedimentation. The depth interval 197.1–235.8 mbsf was not cored.

Unit II

Interval: Section 301-U1301-17H-1, 0 cm, through 19H-CC, 3 cm

Depth: 235.8–265.3 mbsf

Lithology: hemipelagic clay sequence

The lowermost 30–40 m of the sedimentary section overlying basaltic basement consists of homogeneous hemipelagic dark greenish gray clay (Fig. F18). Very thin (<2 cm) fine sand layers are present in the upper portion of Unit II and are of similar composition to the thin sand layers in Unit I. Chlorite and iron oxide form a small proportion of the clay, and fine-grained organic matter was also identified (Fig. F17). There are rare carbonate nodules or pipes and siliceous concretions within this unit, which is slightly bioturbated. No basement rocks were recovered from below Unit II.

Completeness of sediment section recovered from Hole U1301C

Although we do not know the exact depth of basement below Hole U1301C, there are three good indications that we sampled the deepest part of the sedimentary sequence. First, seismic data show sediment cover in the vicinity of Hole U1031B of 290 ms, whereas the uppermost basement reflector in Hole U1301C is found at 280 ms below the seafloor (Fig. F3). Sediment thickness was 265.2 m in Hole U1301B, and Hole U1301C penetrated to 265.3 mbsf (core recovery extending to 262.9 mbsf), consistent with the sediment thicknesses indicated by seismic data.

Second, we recovered a hemipelagic unit at the base of Hole U1301C (Unit II) that was at least 27 m thick; only 12 m of the equivalent unit was recovered in nearby Hole 1026C. The basal hemipelagic clay unit was 40 m thick at Site 1027, located above a buried basement trough 2.2 km to the east, and experience from Leg 168 shows that this unit is typically considerably thinner over basement highs. It is unlikely that Unit II is significantly thicker than the 27 m that was recovered.

Finally, geochemical data from pore fluids squeezed from the sediments of Unit II show that these fluids are (essentially) geochemically identical to those at the base of Hole 1026C and to Baby Bare outcrop vent and seep fluids, particularly with regard to major elements (see "Biogeochemistry"). Pore fluids recovered from the base of Hole U1301C are in geochemical equilibrium with the underlying basement; we see no evidence for significant gradients in major element fluid composition that would suggest a substantial missing sediment section. There were differences in the fluid chemistry of Hole 1026C and U1301C samples for some trace elements and metals, but this discrepancy results mainly from differences in coring and sample handing techniques (see "Biogeochemistry").

Collectively, these observations and inferences suggest that there is no more than 1–2 m of additional sediment below the base of the cored interval of Hole U1301C, and, in fact, we may have recovered the deepest sediment in this location.

Deformation

Five of the cores (301-U1301C-15H through 19H) have distorted and brecciated zones at the top of Section 1. These zones contain mud/clay clasts within a disrupted muddy matrix, along with granules and pebbles similar to those observed in cores from Subunit IC (Fig. F19). Some clay clasts have well-developed slickenlines. These are not considered to be primary deformation structures and instead relate to a change in the APC coring method toward the bottom of the hole. To increase the distance of APC penetration into the deep, compacted sediments, the piston core was pulled back slightly from its previous maximum penetration depth and given a "running start" into the sediment (see "Operations"). The brecciation of the upper 80 cm of the lowermost cores may be a result of the APC impact, with granules probably falling in from above. No primary deformation structures are described in Hole U1301C.

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