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doi:10.2204/iodp.proc.318.106.2011

Lithostratigraphy

Hole U1358A was drilled to a total depth of 2.0 mbsf and Hole U1358B to a total depth of 35.6 mbsf, both using the RCB system. The upper 8.2 m is unconsolidated and moderately to strongly disturbed by drilling (Cores 318-U1358A-1R and 318-U1358B-1R). Below 8.2 mbsf (Cores 318-U1358B-2R through 4R), the sediments are consolidated and only slightly disturbed by drilling. Based on visual core descriptions and smear slide analyses, Holes U1358A and U1358B penetrated diamictons and diamictites (Fig. F3) and are placed within a single lithostratigraphic unit.

Unit description

Unit I

• Interval: 318-U1358B-1R-1, 0 cm, through 4R-1, 19 cm

• Depth: 0–28.57 mbsf

• Age: Pliocene–Pleistocene

Unit I consists of diamicton and diamictite that transitions between clast-rich muddy and clast-rich sandy composition. The uppermost cores from both holes (Cores 318-U1358A-1R and 318-U1358B-1R) consist of unconsolidated light brownish gray massive clast-rich muddy diamicton that is moderately to strongly disturbed by drilling. Diatoms in trace abundances were identified in smear slides of these cores (see Site U1358 smear slides in “Core descriptions”). Cores below 8.2 mbsf (Cores 318-U1358B-2R through 4R) consist of consolidated greenish gray to gray massive to crudely stratified clast-rich sandy and muddy diamictite. A distinct color change from greenish gray to gray was observed between Cores 318-U1358B-2R and 3R. Below interval 318-U1358B-3R-3, 15 cm (20.45 mbsf), the diamictite is sparsely stratified, with few planar but horizontal to inclined laminations. Clast abundance was visually estimated at between 5% and 7.5%, and clasts are as large as 24 cm. Clast lithologies are variable and include basalt, granitic gneiss, quartzite, and fine-grained metasediments. Clasts have subangular to subrounded shapes and basalt and metasedimentary clasts are often faceted and occasionally striated and fluted.

Interpretation

The diamictons in the upper 8.2 mbsf were probably deposited from floating ice. The diamictites below 8.2 mbsf were either deposited from floating ice (where crudely stratified and laminated) or subglacially, with possible remobilization through glacigenic debris flow.

Clay-mineral X-ray diffraction analysis was performed on only one sample (318-U1358B-3R-1, 10–11 cm), which may not be representative for the entire unit. This sample is derived from a clast-rich sandy diamictite facies, and the clay mineral assemblage is dominated by illite (~80%) and chlorite (~15%). A minor component of smectite (~5%) and a trace amount of kaolinite (<1%) are also present in the sample. The predominance of illite and chlorite is indicative of active physical weathering of granitoid and metamorphic parent rocks in a glacial regime. Similar clay mineral assemblages are typical of Pliocene–Pleistocene sediments of the Wilkes Land margin (e.g., Damiani et al., 2006).

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