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Site U13551

Expedition 318 Scientists2

Site summary

The primary objective at Integrated Ocean Drilling Program (IODP) Expedition 318 Site U1355 (proposed Site WLRIS-06A) was to core across unconformity WL-U3 to obtain the timing and nature of the first arrival of the ice sheet to the Wilkes Land Continental Margin in a distal setting. Site U1355 is at the transition between the continental rise and the abyssal plain in a water depth of 3723 meters below sea level (mbsl) (Fig. F1).

Multichannel seismic reflection profiles crossing Site U1355 (Fig. F2) image three of the Wilkes Land margin regional unconformities (WL-U3, WL-U4, and WL-U5) (see the “Expedition 318 summary” chapter). Unconformity WL-U3 is observed between ~782 and 825 meters below seafloor (mbsf) (5.95 ms two-way traveltime [TWT]) and was interpreted to separate preglacial strata below from glacial strata above. Thus, coring across unconformity WL-U3 was intended to document the first arrival of the ice sheet to the Wilkes Land Continental Margin. This “onset” of glaciation is presently inferred to have occurred during the earliest Oligocene (see the “Expedition 318 summary” chapter).

Site U1355 was also chosen to provide a distal record of Oligocene–Pliocene(?) glacial–interglacial (i.e., colder versus warmer) and ice sheet variability. Regional unconformity WL-U5 is imaged in the seismic data at ~709 mbsf (5.6 s TWT) (Fig. F2). Unconformity WL-U5 represents a major shift in continental rise sedimentation with the onset of thick levee deposits above the unconformity. Coring across unconformity WL-U5 aimed to document the timing, nature, and cause of this shift in sedimentation.

Guided by the regional seismic interpretations, Site U1355 is where the uppermost sedimentary section is relatively thin, or has been eroded, so that unconformity WL-U3 could be reached at a shallower depth in contrast to other locations offshore the Wilkes Land margin.

Based on the seismic facies at Site U1355, the lithologies expected were fine-grained distal turbidites, contourites, and hemipelagites (Escutia et al., 1997, 2000, 2002; De Santis et al., 2003; Donda et al., 2003). This interpretation was supported by the sediments recovered from Deep Sea Drilling Project Leg 28 Site 269 (Hayes, Frakes, et al., 1975) located on the abyssal plain ~280 km seaward from Site U1355.

Four cores from one hole were obtained at Site U1355. Cores 318-U1355A-1R through 4R penetrated from 0 to 31.7 mbsf and recovered 14.95 m (47%). The stratigraphic integrity of most of the core was highly compromised by drilling disturbance. The sediments are composed of angular igneous and metamorphic fragments. These fragments are unconsolidated, clast-supported, moderately to well-sorted sandy granule–pebble conglomerates grading upward into well-sorted, fine, crudely stratified sands. One 3 cm thick interbed of dark greenish gray diatom-bearing silty clay was preserved between two upward-fining units. The mechanism for the formation of the upward-fining beds is through gravity flow, most likely a high-density turbidity current.

Samples from Hole U1355A were analyzed for siliceous microfossils, foraminifers, and palynomorphs. Core catcher samples from Cores 318-U1355A-1R through 4R and additional samples from clay-rich clasts within the cores were analyzed for diatoms. The core material yielded an abundant Antarctic flora dominated by Fragilariopsis kerguelensis and Thalassiosira lentiginosa. The association of these typical Pleistocene–Holocene Antarctic diatoms along with common Actinocyclus ingens and Actinocyclus ingens var. ovalis indicates an age no older than late Pleistocene. Reworking from Miocene and Eocene material was recorded. A sample from the top of the hole yielded a rich and diverse modern (Holocene) Antarctic diatom assemblage. Radiolarians typical of late Pleistocene–Holocene Antarctic waters were also found in the core catchers and seafloor samples with an overall low abundance. The seafloor sample yielded a low-diversity planktonic foraminifer assemblage dominated by Neogloboquadrina pachyderma, indicating an age <9.2 Ma. Palynomorphs were recorded in the seafloor sample and Samples 318-U1355A-1R-CC and 4R-CC. Notable finds included Holocene organic-walled dinoflagellate cysts (dinocysts), foraminifer linings, copepod eggs, reworked late Eocene dinocysts, and reworked Paleogene and/or Cretaceous spores and pollen.

The physical property program for Hole U1355A cores included nondestructive measurements of gamma ray attenuation (GRA) densitometer bulk density, magnetic susceptibility, natural gamma ray (NGR) emission, and P-wave velocity on whole-round core sections. Whole-round and section-half core logging measurements are significantly affected by poor core quality, and the data are therefore compromised. Magnetic susceptibility values are relatively high, reflecting the lithologic composition of the individual clasts in the gravels and sands. The silty diatom-bearing clay clasts are characterized by pronounced lower magnetic susceptibility, bulk density, and sonic velocity values but higher NGR counts. P-wave velocities increase from 1800 m/s at the seafloor to >1920 m/s at the base of Core 318-U1355A-2R.

In summary, operations at Site 1355 revealed that the nature of the seafloor was not the expected fine-grained sediments. After failing to core into the seafloor with the advanced piston corer (APC), the rotary core barrel (RCB) system was tried (see “Operations”) and yielded 31.7 m of Pleistocene–Holocene unconsolidated coarse gravels and sands. The unconsolidated and coarse-grained nature of the sediment prevented any further advance. We decided to abandon Hole U1355A and move to Site U1356, where we could achieve the same scientific objectives.

1 Expedition 318 Scientists, 2011. Site U1355. In Escutia, C., Brinkhuis, H., Klaus, A., and the Expedition 318 Scientists, Proc. IODP, 318: Tokyo (Integrated Ocean Drilling Program Management International, Inc.). doi:10.2204/​iodp.proc.318.103.2011

2Expedition 318 Scientists’ addresses.

Publication: 2 July 2011
MS 318-103