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doi:10.2204/iodp.proc.347.111.2015

Lithostratigraphy

The cores recovered from Site M0067 were retrieved from two holes, M0067A and M0067B, drilled to 9.30 and 10.90 mbsf, respectively. The sediments are slightly disturbed in the uppermost 4 mbsf, and deeper lithologic changes presented coring problems, resulting in major core disturbance and reduced core recovery. This resulted in an open-hole advance in Hole M0067B between 5 and 9.5 mbsf. Based on poor recovery and allocations of the recovered material to either optically stimulated luminescence (OSL) dating or palynology/sedimentology offshore, no more cores were described as part of the Onshore Science Party (OSP) deeper than Sections 347-M0067A-3H-1 and 347-M0067B-5H-2.

According to visual core descriptions, Site M0067 is divided into two lithostratigraphic units (Fig. F1). Unit I (0–3.3 mbsf in Hole M0067A and 0–4.42 mbsf in Hole M0067B) is composed of a faintly laminated brown sapropel with millimeter-scale black organic-rich clay laminations and shell fragments, possibly indicating a marine to brackish-marine environment. Unit II (3.3–9.3 mbsf in Hole M0067A and 4.42–10.9 mbsf in Hole M0067B) is a decimeter-scale mixed sand and silt clast–rich unit that most likely represents glacial proximal deposit, with variations associated with ice advance and retreat stages.

Unit I

• Intervals: 347-M0067A-1H-1, 0 cm, to 2H-1, 0 cm; 347-M0067B-1H-1, 0 cm, to 2H-2, 38 cm
• Depths: Hole M0067A = 0–3.3 mbsf; Hole M0067B = 0–4.42 mbsf

This unit is composed of brown-gray well-sorted clay. The unit is mostly homogeneous with faint millimeter-scale lamination of organic-rich clay and minor bioturbation. There is a strong smell of H₂S, suggesting the presence of sulfidic elements within the core. Some shell fragments and other bioclasts are present. Core sections were generally well recovered with only slight to occasionally moderate disturbance, particularly at the top of the hole.

The biogenic sulfidic content and low terrigenous fraction of the sediments in Unit I indicate sedimentation in a high-productivity environment and/or a low supply of terrigenous sediment. The fine lamination with an imprint of minor bioturbation and the generally homogeneous character of this brown laminated clay are interpreted as evidence for the establishment of marine to brackish-marine conditions, with low oxygen, to encourage organic matter accumulation.

Unit II

• Intervals: 347-M0067A-2H-1, 0 cm, to end of hole; 347-M0067B-2H-2, 38 cm, to end of hole
• Depths: Hole M0067A = 3.3–9.3 mbsf; Hole M0067B = 4.42–10.90 mbsf

A gradational boundary from Unit I marks a transition into a gray mixed sediment unit, comprising medium- to coarse-grained sand, silt, and minor clay showing moderate disturbances (Fig. F2). Pebbles ranging from angular to subrounded are present and are up to 1 cm in size. Some clay-rich, centimeter-scale interbeds are present. Clast content increases toward 6 mbsf, where it grades into a poorly sorted gray coarse sandy conglomerate followed by a gravel conglomerate. Clasts include flint, crystalline clasts, limestone, and sandstone. From ~6.3 mbsf, there is a medium-grained, massive gray sand with minor silt content. It is well sorted with biotite, white mica, opaques, and common reworked bioclasts. Around 6.50 mbsf are millimeter- to centimeter-scale black to dark gray crystalline clasts. Clast content decreases greatly deeper than this. The cores in the middle and lower part of this unit are heavily waterlogged, with some mass loss.

The moderately to poorly sorted character of sediments, the polymict clast assemblage, and the abrupt shifts in lithologies in the upper part may indicate deposition in an ice-proximal depositional environment. One possibility may be a proximal outwash plain where braiding of meltwater channels could account for the variation in lithology. Based on the well-sorted nature of the sand in the middle and lower part of Unit II, these deposits may represent a high-energy, fluvial, or deltaic depositional environment, possibly an outwash plain.

The high clast content in the upper part may mark the onset of glacial-proximal deposition. Alternatively, based on the high core disturbance, the clasts may have been transported downhole during the coring process. This would explain the high clast content at the top of the unit and the fewer dispersed clasts lower in the unit and along the cavities at the edge of the core liner.

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