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doi:10.2204/iodp.proc.318.104.2011 Site U13561Expedition 318 Scientists2Site summaryIntegrated Ocean Drilling Program Site U1356 (proposed Site WLRIS-07A) is located at the transition between the continental rise and the abyssal plain at 3992 meters below sea level (Fig. F1). The main objective at Site U1356 was to core across regional 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. Similar to Site U1355, three regional unconformities reported from the Wilkes Land margin (unconformities WL-U3, WL-U4, and WL-U5) (see the “Expedition 318 summary” chapter) are imaged at Site U1356 (Fig. F2). Unconformity WL-U3, interpreted to separate preglacial strata below from glacial strata above and presently inferred to have formed during the earliest Oligocene, occurs at ~867 meters below seafloor (mbsf) (6.33 s two-way traveltime [TWT]). Unconformities WL-U4 and WL-U5 are imaged at ~708 and 534 mbsf (6.15 and 5.95 s TWT), respectively, based on assumed velocities (Fig. F2). Coring across unconformities WL-U4 and WL-U5 was to provide a distal early Oligocene–(?)Pliocene record of glacial–interglacial (i.e., colder versus warmer) and ice sheet and sea ice variability. Multichannel seismic reflection profiles crossing Site U1356 indicated that the site would penetrate a thick sequence of stacked levee deposits developed between two deep-sea channels and above unconformity WL-U4 (Fig. F2). Based on their geometry and seismic facies associations, these levee deposits are interpreted to form when high volumes of sediment were delivered to the continental shelf edge by a wet-based East Antarctic Ice Sheet (Escutia et al., 2000). Reworking of turbidite deposits by bottom-water currents was also suspected based on the presence of wavy reflectors (Escutia et al., 2000, 2002; Donda et al., 2003). Drilling at Site U1356, therefore, would examine the earlier stages of development of large sediment levees denoted by unconformity WL-U5 in addition to the drivers for this change. Below the levee deposits and closely coinciding with unconformity WL-U4, strata are characterized by horizontal and continuous reflectors of varied amplitude that are interpreted to represent hemipelagic and distal turbidite and contourite sedimentation, based on comparisons with sediments recovered at Deep Sea Drilling Project (DSDP) Leg 28 Site 269 (Hayes, Frakes, et al., 1975). Hole U1356A was cored to 1006.4 mbsf. The dominant lithofacies are moderately to strongly bioturbated claystone and calcareous claystone with Zoophycos or Nereites ichnofacies. Subordinate lithofacies include laminated silty claystones, diamictites, mudstones and sandstones with dispersed to common clasts, and graded or cross-laminated siltstones and sandstones. These facies associations are interpreted to result from hemipelagic sedimentation with variable bottom current and gravity flow influence. Wavy submillimeter-thick black concretions observed below ~373 mbsf are interpreted as a form of silica diagenesis. Carbonate-cemented sandstones and conglomerates are also present below this depth. The sedimentary section is divided into 11 lithostratigraphic units (Fig. F3).
Siliceous microfossils (diatoms and radiolarians), calcareous nannofossils, and organic-walled dinoflagellates (dinocysts) are the primary source of microfossil-based age control for Hole U1356A (Fig. F4). The different microfossil groups resolve the stratigraphy nearly exclusive of one another by depth. Diatoms provide a high-resolution stratigraphy for the uppermost lower Miocene to the lowermost Pliocene drape (the upper 387 mbsf). Calcareous nannofossils and, to a lesser extent, dinocysts resolve the Oligocene interval between ~434.5 and ~875.12 mbsf. Dinocysts provide the only microfossil age control for the lowermost Oligocene and the Eocene (~895.5 and ~995.32 mbsf). Radiolarians and planktonic foraminifers provide secondary age control, which is in agreement with the other fossil groups. Diatoms and radiolarians suggest that the Pleistocene and all but the lowermost Pliocene (i.e., 0 to ~4.2 Ma) are missing from Hole U1356A and that at least two other major hiatuses are centered around 4.6 and 28.38 mbsf. This indicates that much of the upper Miocene is also missing. A break in sedimentation near the Oligocene–Miocene transition is suggested by foraminifers and nannofossils. The duration of this hiatus is ~6 m.y., between 23.12 and 17.2 Ma, with most of the early Miocene missing. A relatively thick (~400 m) Oligocene sequence is constrained by magnetostratigraphy and partially by dinocyst and nannofossil data. The very lowermost Oligocene, the upper Eocene, and most of the middle Eocene are missing in a long hiatus from 47.9 to 33.6 Ma based on dinocyst evidence. The oldest break in sedimentation occurs in lower Eocene strata between Sample 318-U1356A-100R-1, 100 cm, and 101R-1, 100 cm (940.10–949.80 mbsf). Paleomagnetostratigraphy and dinocyst stratigraphy indicate this hiatus spans from 51.90 to 51.06 Ma, practically coinciding with the boundaries between lithostratigraphic Units X and XI at 948.8 mbsf. Three intervals with continuous enough recovery and sufficient biostratigraphic data establish a correlation to the geomagnetic polarity timescale (GPTS) (Table T1). Cores 318-U1356A-14R through 51R appear to correlate to polarity Chrons C5AAn to C5Cn.3n. A mid-Miocene hiatus could be placed just above Core 46R because the polarity stratigraphy from Cores 46R through 51R does not have a straightforward fit to Chrons 5Dn and below. Cores 68R through 92R correlate to polarity Chrons C7An–C12R. The lowermost normal polarity interval in Core 105R and the top part of 106R corresponds to Chron C24n.3n based on biostratigraphic evidence. Core 104R is dominantly reverse polarity with a short normal polarity interval correlating to Chron C24n.2n, also based on biostratigraphic data. The top of Core 104R and all of 103R record Chron C24n.1n. Whereas Core 102R has reverse polarity and fits within Chron C23r, Core 101R could record the base of Chron 23n.2n. The correlation of Cores 100R and 99R is not straightforward because they mostly have reverse polarity, whereas Chron C23n.2n is of normal polarity. Because the transition recorded in Core 101R and correlation to Chron C23n.2n fits with a constant sediment accumulation–rate model extrapolating upward from the Chron C24n tie points, the hiatus is most likely positioned between Cores 101R and 100R. The smallest gap would place the transition recorded in Core 99R at the base of Chron C23.1n with Core 100R being Chron C23r in age, with a sliver of Chron C23.2n at its base (interval 318-U1356A-101R-1, 100 cm). This magnetostratigraphic interpretation matches the dinocyst stratigraphy and the lithostratigraphy. Carbonate contents for most of the section are below 2 wt%, and levels of organic carbon, sulfur, and nitrogen are below detection limits. Major and trace elements (SiO2, Al2O3, TiO2, K2O, Na2O, MgO, Fe2O3, P2O5, Sr, Ba, Sc, Co, and V) show pronounced downcore variations, which can be summarized in three geochemical intervals:
Physical properties generally change at the identified lithostratigraphic boundaries at Site U1356. Velocity, density, and porosity data clearly reflect the positions of features such as unconformities WL-U5 and WL-U3. The grain density increases from ~2.6 g/cm3 at the seafloor to ~2.8 g/cm3 at 1000 mbsf. Magnetic susceptibility data exhibit rhythmic changes, which are especially visible in the cores with improved recovery starting at Core 318-U1356A-47R and moreso from Core 68R downward. |
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