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doi:10.2204/iodp.proc.347.108.2015 LithostratigraphyAt Site M0064, four holes were drilled: Hole M0064A to a total depth of 41.5 mbsf, Hole M0064B to 10.2 mbsf, Hole M0064C to 45.10 mbsf, and Hole M0064D to 41.2 mbsf. The top ~12 to ~15 m in each hole was retrieved by piston coring with nearly 100% recovery in Holes M0064A, M0064C, and M0064D. Hole M0062B terminated in a sand and gravel deposit (Fig. F1). Deeper than ~15 mbsf, the nonrotating core barrel was used to ~35 mbsf, with ~50% recovery in each hole, encountering different formation depths within each hole. A combination of nonrotating core barrel, push coring, open holing, and hammer sampling achieved advancement of the drill bit with nearly 100% recovery between ~35 and 38 mbsf in Holes M0064A and M0064C but with limited recovery deeper than this depth (see “Operations”). Lithostratigraphic divisions (Units I–IV; Fig. F2) are based on descriptions on the cut face of the split core and observations from smear slides (see “Core descriptions”). Units I and II are absent in Holes M0064B and M0064C. Unit I
Unit I is very dark greenish gray diatom-bearing clay, 0.33 to 1.34 m thick, largely homogeneous, but with weak parallel planar laminations at the base of the unit (Fig. F3). Laminae are black in color, and a smear slide from the base of the unit reveals a relatively large amount of opaque, possibly iron sulfide, minerals. This unit was likely deposited in a brackish marine or lake environment with low terrigenous sediment input. For the base of the unit, a stratified water column, low oxygen, and a lack of benthic life may have allowed for preservation of lamination. Accumulation of organic matter may have formed the precursor to the iron sulfide minerals. Unit II
Unit II is greenish gray to very dark greenish gray sandy silt, 0.27 to 0.41 m thick. The silt is stratified with a coarser, more sand-rich upper part (Fig. F3). No fossils were found in the unit. The sediment probably represents deposition in a shallow, oxidized, proglacial lake environment with relatively high terrigenous sedimentation rates. Unit IIISubunit IIIa
Subunit IIIb
Unit III is dark grayish brown laminated clay and silty clay with dispersed clasts. The clay is rhythmically laminated by color on a centimeter scale and locally contains dispersed gravel clasts, sand, and sedimentary intraclasts. Inclined laminae and synsedimentary microfaults are also observed. Lamination is planar, parallel, and inclined, and in Subunit IIIb, it comprises distinct varve-like fining-upward silty clay to clay couplets. Subunit IIIa is more brown in color, whereas Subunit IIIb is more gray with an increase in silt toward the bottom of the unit. The bottom 3 cm of Subunit IIIa is dark brown and forms the boundary between Subunits IIIa and IIIb (Fig. F3). The contact between Subunits IIIa and IIIb is distinct in color, but there is no change in sediment texture and the lamination has a consistent orientation across the subunit boundary. Deposition in a glacial lake is inferred for this unit. The inclined laminae, synsedimentary microfaults, and sedimentary intraclasts may indicate minor slumping due to an unstable sloping environment or high sedimentation rates. Outsized gravel clasts probably represent ice rafting. Unit IVSubunit IVa
Subunit IVb
Subunit IVc
Unit IV is composed of moderately to poorly sorted coarse-grained sediments with variable gravel abundances and sand content in the matrix. Subunit IVa consists primarily of clast-rich stratified muddy diamicton and sandy gravel. Within the diamicton, contorted mud stringers and laminae, as well dark color banding, are observed (Fig. F3). Subunit IVb is dominated by massive, clast-poor, sandy diamicton (Fig. F3). Occasional potential clast alignment fabrics were observed in this facies. Subunit IVc consists of stratified sandy clayey silt with dispersed clasts and thin intervals of diamicton. In this latter subunit, a large variation in clast abundances is observed over relatively short depth intervals. Unit IV was deposited in an ice-influenced depositional environment. Subunit IVa shows evidence of meltwater flow and minor gravity flow, as well as features of an ice-contact outwash plain. Subunit IVb probably represents an ice-proximal deposit with massive diamicton originating from either subglacial processes, glacigenic debris flow, or glaciomarine/glaciolacustrine deposition. Subunit IVc likely originated in an ice-influenced aquatic depositional environment, with variations in clast abundances due to changes in glaciomarine or glaciolacustrine deposition. |
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