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doi:10.2204/iodp.proc.318.108.2011 LithostratigraphyOnly 60 cm of sediment was recovered in the upper 14.3 mbsf of Hole U1360A (Core 318-U1360A-1R) The recovered sediments are unconsolidated and moderately disturbed by drilling. Below 14.3 mbsf (Cores 2R through 6R), 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 descriptionsUnit I
Unit I 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 at 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 up to 25% and clasts are primarily composed of angular, indurated olive-green to olive-brown mudstone fragments 2–8 mm in size. Crystalline rock clasts as large as 7 cm, including basalt and gneiss, are also present and have subrounded and faceted shapes. InterpretationThe unconsolidated diamictons were probably deposited from floating ice and likely represent deposition from a floating glacier tongue or icebergs releasing debris over the site. The lamina of diatom ooze may be indicative of a brief period of open-marine conditions with high productivity and low terrigenous sedimentation rates. Unit II
The top of Unit II marks a sharp change in lithology and induration of the core, from unconsolidated diamicton above to carbonate-cemented claystone below. The strata in Unit II are generally well indurated, and carbonate-cemented beds are common. 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. Clast and sand percentages decrease upsection throughout the unit. Bivalve shell fragments, some of which are pyritized, are common in the lower portion of Unit II, below interval 318-U1360A-4R-1, 48 cm. Clast lithologies are mainly gneiss, granite, diorite, and quartzite. InterpretationThe lithofacies distribution present in Unit II 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). Clay mineralogyFive samples from Hole U1360A were prepared for XRD analysis of the clay fraction (i.e., one sample per core from Cores 318-U1360A-1R, 3R, 4R, 5R, and 6R). The clay mineral assemblages in these samples are characterized by a mixture of all major clay mineral groups. The dominant clay-mineral components are smectite, illite, and chlorite, with a lesser contribution of kaolinite and pyrophyllite-talc (Fig. F7). Cores 318-U1360A-3R through 6R are assigned an early Oligocene age (see “Biostratigraphy”), and the clay mineral assemblages in these cores are 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 is consistent with a glacial-marine depositional setting for the claystone, mudstone, sandstone, and diamictite facies described within 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. |
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