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doi:10.2204/iodp.proc.347.103.2015 LithostratigraphyCores were recovered from Holes M0059A–M0059E. The sediments are slightly disturbed in the uppermost 85 mbsf, and lithologic changes deeper than this point presented coring problems, resulting in major core disturbance and low core recovery (see “Operations”). These issues are in part due to piston coring for the uppermost 85 mbsf before a switch to a combination of open hole and hammer sampling because the piston corer could no longer penetrate the lithology, which consisted of successions of coarse-grained diamicton and gravely sands. Deeper than 169 mbsf, coring encountered limestone bedrock, and Hole M0059B was terminated (204 mbsf). The dominant lithofacies in the uppermost 80 mbsf are clay deposits with subordinate lithofacies that include laminated organic-rich clay, silty clay, and semivarved clay with downhole increases in silt and sand content as well as the rare occurrence of pebble-sized clasts. Based on visual core descriptions and smear slide analyses, Site M0059 is divided into seven lithostratigraphic units (Fig. F1). Subunit Ia (0–47.1 mbsf; Hole M0059A) is composed of laminated, organic-rich biosiliceous clay, indicating a marine to brackish environment. The subunit is influenced by the effects of methane expansion. Subunit Ib (47.1–51.68 mbsf; Hole M0059A) is a downhole continuation of Subunit Ia, but silt in the biosiliceous clay and the presence of freshwater diatoms show a change to freshwater conditions. Unit II (51.68–51.73 mbsf; Hole M0059A) is a centimeter-scale silty sand unit that reveals an erosional unconformity. Unit III (51.73–82.20 mbsf; Hole M0059A) is characterized by rhythmites of clayey silt and silty clay with low organic content and a downhole increase of sand and rare dispersed gravel-sized clasts, which is evidence for ice rafting in a glaciolacustrine environment. Unit IV (82.20–93.12 mbsf; Hole M0059B), Unit V (93.12–99.12 mbsf; Hole M0059C), and Unit VI (99.12–169.03 mbsf; Hole M0059B) comprise a variety of gravel-bearing lithologies and interlayered glacial clays, proximal glacial sands, and diamicton indicative of a succession of repeated glaciations, followed by the final deglaciation that continues uphole in Unit III. Unit VII (169.03–204.03 mbsf; Hole M0059B) is the lowermost unit at Site M0059, and despite very poor recovery, limestone with flint gravel, moderately consolidated, and rich in bioclastic material indicates penetration of Cretaceous limestone bedrock. Unit ISubunit Ia
This uppermost subunit is composed of black to greenish black well-sorted clay. The subunit is mostly homogeneous with faint millimeter-scale lamination and minor bioturbation (Fig. F2). Some shell fragments and other bioclasts are present and increase in abundance in the lower half of the subunit. Core sections were generally well recovered with only slight to occasionally moderate disturbance. It is estimated from smear slides that the major lithology is biosiliceous clay with 64%–89% clay minerals and 5%–27% biogenic silica (primarily diatoms, silicoflagellates, and sponge spicules). The high biogenic and low terrigenous contents of the sediments in Subunit Ia indicate hemipelagic sedimentation in a high-productivity environment and/or a low terrigenous sediment supply. The fine lamination with an imprint of minor bioturbation and the generally homogeneous character of this black organic-rich clay are interpreted as strong evidence for the establishment of marine conditions, with low-oxygen bottom water during the Holocene referred to as the Littorina transgression (i.e., the marine transgression at the onset of the Littorina Sea stage of the Baltic Sea). Based on smear slide analysis, the isolated light-colored laminae noted within this subunit constitute fibrous diatomaceous mats interpreted to represent biosiliceous bloom events along with silicoflagellates, also indicative of a marine environment. Subunit Ib
This subunit contains greenish gray well-sorted silty clay and is generally well preserved with only slight disturbances. The subunit has more prominent pale green laminae that tend to occur over centimeter scales instead of the fine millimeter-scale lamination of the subunit above. This difference is due to a terrigenous silt component. Estimated from smear slides, the major lithology is biosiliceous silty clay with a quartz silt component. No bioturbation is visible in this subunit. Core disturbance is only slight with some degradation and oxidation along the core liner and ends. Based on smear slides, freshwater diatoms are present in the main lithology but are more abundant in light green to pale brown mats composed of colonial diatom chain fragments, indicating diatom bloom and lake dump events (see “Biostratigraphy”). Coupling this observation with the nature of the fine-grained millimeter- to centimeter-scale color laminations suggests that, overall, Subunit Ib represents the establishment of a freshwater lake stage. Unit II
Unit II is a light yellowish brown, poorly sorted, centimeter-scale silty sand horizon (Fig. F3). Subangular and rounded quartz is present, acting as the grain support to the granule- to pebble-sized clasts of various lithologies, overall exhibiting inverse grading. More than 10 clasts >3 mm in length were counted in this small horizon, primarily at the upper contact. Because of the sharp but irregular basal contact, indicative of a downward-cutting erosional surface that gradually increases in grain size toward the top, this unit marks a lithostratigraphic boundary. This horizon may represent an erosional unconformity, marking a potential lowstand. Unit III
Unit III is greenish gray clay with centimeter-scale color banding and centimeter-scale laminations. Laminae consist of well-sorted quartz silt, which tends to increase in abundance toward the base of the unit. Millimeter- to centimeter-scale clasts of varying angularity are observed in intervals in the lower half of the unit. A generally fining-upward sequence is seen, based on an increase in silty fine sand laminations increasing in frequency toward the base of the unit. Evidence of convolute bedding and soft-sediment deformation is observed in the lower part of the unit. Estimated from smear slides, rare angular and subangular sand grains are present, as well as traces of broken diatoms and other siliceous debris. The rhythmically banded clays are indicative of lake deposits. Observed subangular sand and centimeter-scale pebble grains are interpreted to be dropstones originating from ice rafting, which suggests that the unit most likely represents a late glacial ice lake. Traces of broken diatoms and other siliceous debris suggest reworking. The increased grain size and frequency of sand laminations in the lower sections (70.0–82.2 mbsf) suggest an ice-proximal location, with a gradual upward shift toward ice-distal facies at the top of the unit. The convolute bedding and soft-sediment deformation located in the lower part of the unit are suggestive of slumping; however, they also may not be primary sedimentary features. They may instead be the result of pressure differentials and fluid liberation within the waterlogged clays during drilling (Fig. F4). Unit IV
Olive-gray silty clay occurs at the top of this unit and gradually grades downward into a series of centimeter-scale sandy laminations with increasing detritus toward the base as evidenced by more thickly bedded, poorly sorted sand units. Gray sand with silty clay intraclasts and a large bioclastic content is principally composed of mollusk, coral, and echinoderm fragments. Both the sand laminae and the massive sand unit are moderately well sorted sand with quartz and feldspar supporting grains, with biotite, amphiboles, garnet, rounded glauconite, reworked foraminifers, diatoms, and other organic debris. A massive fine-grained horizon deeper in this unit is well-sorted silt with a minor sand fraction, which has very similar composition to the sands in this unit. The variation between mature and less mature lithic-bearing sands with clay-rich horizons and reworking of material is suggestive of glacial outwash deposits. This variation may represent a braided outwash network accounting for shifts between sand- and silt-dominated horizons. Unit V
Unit V is dark gray to dark grayish brown poorly sorted clast-poor to clast-rich muddy diamicton. This unit is highly consolidated. Core recovery in this unit was much lower than in the overlying units. However, based on core catcher descriptions coupled with intervals of recovered core, we interpret this interval as a massive diamicton unit. The silt-sand matrix, the poor sorting, and the angularity of the clasts point to a diamicton, possibly representing a glacial till deposit, drawing associations between this and the last period of ice advance in the area. Unit VI
Core recovery in this unit was very poor, with few complete cores described. To fill in some of our missing intervals, core catcher observations were included to interpolate the variation within and boundaries of the unit. Based on the various data, the unit comprises a series of interbedded centimeter- to decimeter-scale silt-rich sequences, as well as coarse sands and horizons with a diamicton character. Despite limited recovery in this unit, the unit is interpreted as an interbedded glacial deposit based on the variation between silt, sand, and diamicton-rich horizons. The variation in lithology could be related to ice advance and subsequent deposition of coarse-grained, poorly sorted material during retreat or melting periods. Unit VII
Unit VII is limestone with flint gravel, moderately consolidated and rich in bioclastic material, including echinoderms, coral, and foraminifers. Based on the findings of Cretaceous foraminifers with no apparent reworking, the unit consists of Cretaceous limestone. |
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