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doi:10.2204/iodp.proc.318.108.2011 Site U13601Expedition 318 Scientists2Site summaryIntegrated Ocean Drilling Program Site U1360 (proposed Site WLSHE-09B) is on the continental shelf off the Adélie Coast (Fig. F1) at 495 meters below sea level. The main objective at Site U1360 was to core across regional unconformity WL-U3 to determine the timing and nature of the first arrival of the ice sheet to the Wilkes Land Continental Margin. Site U1360 lies at the eastern edge of the Adélie Bank and receives drainage from the East Antarctic Ice Sheet (EAIS) through the Wilkes subglacial basin (see the “Expedition 318 summary” chapter). Glacier ice and the entrained debris draining through the basin and extending to the continental shelf would provide evidence for a large-scale ice sheet on Antarctica. Regional unconformity WL-U3 was interpreted to record the first expansion of the EAIS across the shelf in this sector of the East Antarctic margin and therefore to separate preglacial strata below from glacial strata above (Eittreim et al., 1995; Escutia et al., 1997; De Santis et al., 2003). Drilling in Prydz Bay during Ocean Drilling Program (ODP) Leg 188 (O’Brien, Cooper, Richter, et al., 2001) and results from Deep Sea Drilling Project Leg 28 Site 269 (Hayes, Frakes, et al., 1975), ODP Leg 189 sites in the Tasman Gateway (Exon, Kennett, Malone, et al., 2001), and ODP Leg 182 Site 1128 in the Great Australian Bight (e.g., Mallinson et al., 2003), among others, led Escutia et al. (2005) to postulate an early Oligocene age (i.e., 33.5–30 Ma) for the development of unconformity WL-U3. At Site U1360, unconformity WL-U3 was predicted to occur at ~165 meters below seafloor (mbsf) (0.81 s two-way traveltime) (Fig. F2). Multichannel seismic reflection profiles crossing Site U1360 show gently dipping strata on the shelf truncated near the seafloor (Fig. F2). This provides a unique opportunity to sample across the unconformity with very shallow penetration. The proximal record from Site U1360 will complement the distal record of the first arrival of the EAIS to the Wilkes Land margin obtained at Site U1356. Hole U1360A was drilled to a total depth of 70.8 mbsf using the rotary core barrel (RCB) system. Only 60 cm was recovered in the upper 14.3 mbsf, and sediments are unconsolidated and moderately to strongly disturbed by drilling (Core 318-U1360A-1R). Below 14.3 mbsf (Cores 2R through 6R), the sediments are consolidated and most recovered intervals are only slightly disturbed by drilling. Based on visual core descriptions and smear slide analyses, cores from Hole U1360A are composed of diamictons, mudstones, sandstones, and diamictites that are placed into two lithostratigraphic units. Unit I (0–14.3 mbsf) consists of unconsolidated clast-rich sandy diamicton (Fig. F3). The diamicton is slightly compacted but soft and crudely stratified and includes one lamination of yellowish clay-rich diatom ooze in interval 318-U1360A-1R-1, 18–20 cm. A trace of diatoms is present in the matrix of the diamictite. Rare shell fragments are also present in this unit. Clast percentages are as high as 25%, and clasts are primarily composed of angular, indurated, olive-green to olive-brown mudstone fragments, 2–8 mm in size. Crystalline rock clasts, including basalt and gneiss and as large as 7 cm, have subrounded and faceted shapes. The unconsolidated diamictons were probably deposited from floating ice and most likely represent deposition from a floating glacier tongue or icebergs releasing debris over the site. The lamination of diatom ooze is indicative of a brief period of open-marine conditions with high productivity and low terrigenous sedimentation rates. The top of Unit II (at 14.3 mbsf) marks a sharp change in lithology and induration of the cores, from unconsolidated diamicton above to carbonate-cemented claystone below. An early Oligocene age is assigned to the interval below interval 318-U1360A-3R-1, 8 cm, whereas no age assignment is possible for Core 318-U1360A-2R (see “Biostratigraphy”). Five different lithofacies are recognized in a sequence from top to bottom in this unit:
Overall, Unit II can be characterized as a fining-upward sequence from diamictite at the base to claystone at the top. Bivalve shell fragments, some of which are pyritized, are common in the lower portion of Unit II. The lithofacies distribution is consistent with an ice-proximal to ice-distal glaciomarine depositional environment, similar to that described from the Oligocene and Miocene strata of the Victoria Land Basin, Ross Sea, Antarctica (Naish et al., 2001; Powell and Cooper, 2002). Five samples from Hole U1360A were prepared for X-ray diffraction (XRD) analysis of the clay fraction. A mixture of all the major clay mineral groups characterizes the clay mineral assemblages in these samples. The dominant clay mineral components are smectite, illite, and chlorite, with a lesser contribution of kaolinite and pyrophyllite-talc (Fig. F7). The cores assigned to the early Oligocene have clay mineral assemblages similar to those reported from lower Oligocene shelf strata around the Antarctic margin (e.g., Hambrey et al., 1991; Ehrmann et al., 2005). The abundance of illite and chlorite are consistent with a glacial-marine depositional setting for the claystone, mudstone, sandstone, and diamictite facies described within lithostratigraphic Unit II. The relatively large contribution of talc, however, is not typical of Paleogene sediments on the Antarctic shelf and may reflect the weathering of a low-grade metamorphic facies, derived from a basic or ultrabasic igneous protolith, locally on the Wilkes Land margin or within the Wilkes subglacial basin. Dinoflagellate cysts (dinocysts) and diatoms provide age control for Hole U1360A. They suggest that Core 318-U1360A-1R (0–0.54 mbsf) comprises an uppermost Pleistocene matrix with intraclasts of upper Eocene to lower Oligocene material. An age could not be assigned to the strata between Section 318-U1360A-1R-CC and interval 318-U1360A-3R-1, 8 cm (0.54–23.38 mbsf), because of poor recovery. Samples 318-U1360A-3R-1, 8 cm, to 6R-CC (23.38–53.78 mbsf) are of early Oligocene (<33.6 Ma) age. Sediments within Core 318-U1360A-1R (0–0.54 mbsf) comprise an uppermost Pleistocene matrix with intraclasts of late Eocene to early Oligocene age. Diatoms indicate that during the latest Pleistocene, the shelf at Hole U1360A was not subjected to year-round ice cover but was influenced by seasonal sea ice either directly or indirectly. Combined microfossil analyses allow the interval below 318-U1360A-3R-1, 8 cm (23.38 mbsf), to be assigned to the early Oligocene (<33.6 Ma) with confidence. Since these are derived from the same lithostratigraphic Unit II as strata between 14.3 and 23.38 mbsf, the entire Unit II is considered to be of early Oligocene. Early Oligocene microfossils indicate a shallow-water shelf environment with low salinities and high nutrient levels, likely driven by seasonal sea ice. Sporomorphs may represent reworking from older strata and/or contemporaneous vegetation in the hinterland. Two samples from cores of Hole U1360A (Samples 318-U1360A-4R-1, 15 cm, and 4R-2, 72 cm) have reverse polarity, consistent with the age of Chron C12r as indicated by the biostratigraphy (see “Biostratigraphy”). The fining-upward sequence from diamictites at the base (Core 318-U1360A-6R) to claystones at the top (Cores 2R through 3R) (see “Lithostratigraphy”) is also documented in the general decrease in magnetic susceptibility values from the bottom to the top of the hole. Variations in gamma ray attenuation (GRA) density nicely reflect variations in lithology between clast-rich diamictite, sandy mudstone with dispersed clasts, and claystone. Calculated porosity ranges from 49% to 17% and generally decreases with depth. Grain densities range from 2.62 to 2.7 g/cm3. |
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