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doi:10.2204/iodp.proc.347.106.2015 LithostratigraphySix holes were drilled at Site M0062: Hole M0062A to a total depth of 35.9 mbsf, Hole M0062B to 24.1 mbsf, Hole M0062C to 3.3 mbsf, and Hole M0062D to 21.0 mbsf. Holes M0062K and M0062L were Rumohr cores (<1 m penetration gravity cores) designated for microbiology and paleoceanographic sampling. These cores were not included in the core description at the IODP Bremen Core Repository. Expansion of cores and gas release on recovery resulted in variable core quality in the upper ~15 mbsf. Lithostratigraphic divisions (Units I and II; Fig. F1) are based on descriptions made on the cut face of the split core and observations from smear slides (see “Core descriptions”). Unit ISubunit Ia
Subunit Ib
Unit I comprises very dark greenish gray laminated silty clay. Laminae consist of horizontal, planar, and parallel couplets of light and dark laminae. Subunit Ia is laminated on a millimeter scale and contains two distinct 0.5–1 m thick greenish black intervals between ~6 and 8.5 mbsf. The upper interval has a diffuse top and a sharp bottom at 6.3 mbsf at Sections 347-M0062A-3H-2, 100 cm, and 347-M0062B-3H-2, 50 cm, and at 6.45 mbsf at Section 347-M0062D-3H-2, 15 cm. The lower interval has a sharp top at 7.74–8.03 mbsf at Sections 347-M0062A-4H-1, 83 cm, 347-M0062B-4H-1, 14 cm, and 347-M0062D-3H-CC, 0 cm, and a diffuse bottom. Interlaminated “varve-like” silty clay–clay couplets are observed in Subunit Ib, with light-colored silty clay grading upward into clay within each couplet (Fig. F2). The bottom of Unit I is defined based on the first downcore appearance of sand interbeds. A gradual upcore decrease in couplet thickness is observed within Subunit Ib: couplet thicknesses are 4–10 cm immediately above the Unit I/II boundary, decreasing upcore to 3–4 mm over a ~2 m interval (Fig. F2). The couplets in Unit I, and especially Subunit Ib, have characteristics of glacial lake varves. In estuaries, rhythmites can also result from tidal processes; however, the tidal range in the Baltic Sea is negligible. The boundary between Subunits Ia and Ib may mark a transition from lacustrine to brackish-marine conditions in the Ångermanälven River estuary. The greenish black intervals comprise iron sulfide precipitates, likely resulting from diagenetic activity after deposition and preservation of organic matter from a stratified water column. Unit II
Unit II comprises a well-sorted dark gray fine to medium sand. The upper part includes several fining-upward sequences from medium sand to silt (interval 347-M0062A-7H-1, 93 cm, to 7H-2, 150 cm) as thick as 60 cm, as well as silt laminae and silt interbeds. Toward the base, the unit is dominated by homogeneous beds of fine and medium sand. High mica and opaque mineral contents are observed throughout Unit II. The coarse-grained nature of the sediment and the fining-upward beds suggest that this unit was deposited as a result of traction transport in a glaciofluvial, fluvial, or tidal system, although the latter is unlikely because of the nontidal regime of the Baltic Sea. The fining-upward units probably indicate glaciofluvial or fluvial channel deposits. |
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