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doi:10.2204/iodp.proc.347.105.2015 LithostratigraphySix holes were drilled at Site M0061: Hole M0061A to a total depth of 25.2 mbsf, Hole M0061B to 28.7 mbsf, and M0061C to 23.1 mbsf. Holes M0061K–M0061M were Rumohr cores (<1 m penetration gravity cores) designated for microbiology and paleoceanographic sampling. The Rumohr cores were not included in the core description at the IODP Bremen Core Repository (Germany). Piston coring was the primary coring method used in Holes M0061A–M0061C, with the exception of the bottom of Hole M0061A, where it became difficult to retrieve core as the sediment became sandier and coring changed to hammer sampling. At the base of that hole, a hard layer was reached where only a granite rock ~5 cm in diameter was recovered (see “Operations”). The cores are slightly to moderately disturbed, and in Unit IV, the sediment was waterlogged and deformed, which prevented the clear identification of internal structures. Lithostratigraphic subdivisions are based on descriptions on the cut face of the split core from Hole M0061A with a core recovery of nearly 95% and supplemented by information from Holes M0061B and M0061C. Because of time constraints, only a small number of smear slides from core catchers were examined, and the data should be treated with caution because of core disturbance and contamination affecting core catcher material in general. Site M0061 is divided into four lithostratigraphic units (Fig. F1). Unit I (0–2.30 mbsf; Hole M0061A) is composed of greenish black organic-rich clay influenced by expanding methane. A gradual transition is observed to Unit II, which is divided into Subunit IIa (2.30–6.46 mbsf; Hole M0061A), a dark greenish gray weakly varved clay with weak sulfide banding, Subunit IIb (6.46–6.90 mbsf; Hole M0061A), a downhole continuation of Subunit IIa dominated by black laminae rich in iron sulfide, and Subunit IIc (6.90–8.10 mbsf; Hole M0061A), which is gray clay. Unit III (8.10–14.50 mbsf; Hole M0061A) is dark greenish gray clayey silt with a varvic, rhythmic lamination that gradually grades to the dark gray silty sand and fine sands of Unit IV (14.50–28.70 mbsf; Hole M0061B). Unit IV also contains limited numbers of dispersed pebble-sized clasts. Unit I
Unit I is chiefly composed of a greenish gray to black clay that is only slightly disturbed with good recovery. The uppermost 40 cm is mostly greenish gray, becoming blacker downhole, marking increasing organic matter contribution. The clay comprises weak lamination, with millimeter-scale black laminae and spots, often smeared as a result of the core splitting process. Possible bioturbation is present at some intervals but is often very faint. The clay is very well sorted, with small voids due to methane expansion dispersed along the upper half of the unit especially. Smear slide studies (see “Core descriptions”) show that the terrigenous components consist of 90% silt and only 5% sand, as well as clay with a composition dominated by quartz, limited feldspar, and traces of biotite. Black organic-rich clay with indistinct varves likely signifies deposition in conditions similar to a brackish-marine environment. Visible black banding is likely due to diagenetic sulfidization linked to organic accumulation during events of lower oxygen content. Unit IISubunit IIa
Subunit IIb
Subunit IIc
The upper part of the unit consists of dark greenish gray silty-sandy organic-rich clay with weak gray lamination on millimeter scales, often in 2–5 cm bundles. Sparse bioturbation is observed, and the sediment is very well sorted. In Subunit IIb, black iron sulfide laminae dominate (Fig. F2). Subunit IIc is a lithologic continuation of the clay in Subunits IIa and IIb, but gray and without the presence of iron sulfide. Smear slide studies (see “Core descriptions”) show that the terrigenous components consist of 90% sand and 10% silt with a composition dominated by quartz and feldspar and minor contents of biotite. This fine laminated silty clay with high organic content is interpreted as possible annual sediment laminae (varves) deposited in a localized protected basin. The rhythmic appearance possibly reflects local climatic changes in organic matter accumulation. The black laminated horizon represents increased iron sulfide diagenetic processes within Subunit IIb. This increase may represent periods of low oxygen and increased organic matter deposition, providing sufficient conditions for sulfide formation. The lowermost part (Subunit IIc) is a transition zone in the generally lithologically normal graded fjord sediments. Unit III
Unit III comprises dark greenish gray interlaminated clayey silt and silt, rhythmites, and perhaps varves fining upward within one lamina (silt to clay); lamination is visible by changes in grain size and color with a general thickness of laminae around 1–4 cm. The unit is very well sorted and slightly disturbed, gradually grading downhole to the sand of Unit IV. This fine laminated silty sediment is interpreted as annual sediment laminae (varves) deposited in a locally protected basin. The rhythmic appearance possibly reflects local climatic changes. Unit IV
Unit IV consists of dark gray homogeneous silty sand with clayey silt interbeds on a centimeter scale, grading downhole to medium, washed sand containing brown specks (small plant remnants, opaques, and white mica). Grain shapes are subangular to subrounded. A few dispersed gravel clasts are present. The downhole increase of sand and rare gravel-sized clasts may be evidence for a glacially influenced environment, possibly influenced by meltwater flow. |
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