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doi:10.2204/iodp.proc.347.104.2015 LithostratigraphyTwo holes were drilled at Site M0060. Hole M0060A reached a total depth of 232.50 mbsf, and Hole M0060B was 85.70 mbsf deep. Hole M0060B was designated for microbiology sampling and was subject to whole-round sampling on board the ship prior to core description at the IODP Bremen Core Repository (Germany). In Hole M0060A, piston coring was used for the uppermost ~83 mbsf, where recovery was >90%. Between 83 and 200 mbsf, a combination of piston coring, nonrotating core barrel, and extended nose coring was used to optimize recovery. At ~100–120 and ~147–162 mbsf, a combination of open holing and extended nose drilling was used to advance through hard lithologies, which affected recovery and core quality. Between ~101 and 117 mbsf, all core material recovered (17 cm) was consumed by shipboard sampling. For this interval, no core descriptions exist and shipboard lithologic descriptions are limited to observations on one ~5 cm portion of a core catcher and some washed paleontology samples. Deeper than 200 mbsf, open holing, hammer sampling, and push coring were used to aid penetration through difficult lithologies; however, this produced limited core recovery (see “Operations”). Lithostratigraphic divisions (Units I–VII) (Fig. F1) are based on descriptions of the cut face of the split core of Hole M0060A, augmented by available sections from Hole M0060B and observations from smear slides (see “Core descriptions”). Two of the unit boundaries in Hole M0060B were within core sections sampled for microbiology. For these sections, the boundaries were placed at the bottom of the interval that was sampled for microbiology. Unit I
Unit I is composed of gray, massive, fine to medium thickly bedded sand with common marine bivalve and gastropod shell fragments, including Cerastoderma sp., Macoma baltica, and Turritella sp. Two distinct fining-upward shell-rich beds were found in this unit as well. The sand is generally well sorted, and quartz sand grains are subrounded to rounded. The sand was deposited in a near-shore marine depositional environment. Fining-upward shell-rich beds signal deposition near the wave base; therefore, the approximate bathymetry would be similar to the modern situation. Unit II
Unit II consists of dark greenish gray interlaminated sandy clayey silt and fine–medium sand with dispersed clasts. Sand laminae are 0.5–3 cm thick and occur in packages unequally spaced within the silt. The orientation of the laminae is inclined, and they are deformed as a primary sedimentary structure. Quartz sand grains dispersed within the silt are angular to subrounded, and the sediment is moderately well sorted. Reworked mollusk fragments are found throughout, and reworked diatom fragments are common in smear slides from this unit. Sparse bioturbation is observed near the bottom of this unit between black, presumably iron sulfide, laminae. Gypsum was observed macroscopically and in smear slides. The bottom of the unit is sparsely bioturbated between iron sulfide laminated intervals, possibly due to changing stratification of the water column coupled to salinity changes. The deformation in the upper part of the unit is likely due to slumping and possibly the result of increased sedimentation rate. With the presence of dispersed outsized gravel clasts, the lithologic changes can be interpreted as a prograding, ice-influenced deltaic environment. Unit IIISubunit IIIa
Subunit IIIb
Subunit IIIc
Unit III is characterized by dark grayish brown to gray parallel laminated clay and silt with dispersed clasts. In Subunit IIIa, discrete millimeter-scale silt and fine sand laminae occur as packages of 2–4 laminae and are either well preserved or disrupted, possibly due to loading or bioturbation. Laminae are irregularly spaced and generally 3–6 mm thick, and their abundance increases upward through the unit. Subunit IIIa locally has a reddish hue. Numerous black, possibly iron sulfide, bands are present throughout the unit and become especially prominent in Subunit IIIb (Fig. F2). Subunit IIIc has a minor interlaminated sand component. This unit can be interpreted as an ice-influenced lake or marginal marine environment. Silt laminae in Subunit IIIa may represent bottom current activity. The outsized gravel clasts may have originated from ice rafting from a calving glacier at a distance from the drilled location. The presence of iron sulfide bands within the sediment, especially in Subunit IIIb, may be due to periodic oxygen-poor conditions and a stratified water column, where organic matter may have accumulated to form the precursor to the diagenetic sulfides. Unit IV
Gray interbedded sand, silt, and clay with dispersed clasts and clast-poor diamicton were identified in Unit IV. Both rock clasts and intraclasts are common in this unit, and the strata are intensely folded or contorted. Clast assemblages are polymict. The moderately to poorly sorted character of sediments, the polymict clast assemblage, and the abrupt shifts in lithologies may indicate deposition in an ice-proximal depositional environment. The deformation of the sediments may be due to slumping into an aquatic depositional environment. Unit V
This unit is characterized by black and gray sandy silty clay with dispersed clasts. Mollusk fragments are common, especially Turritella sp. Multiple horizons with shell fragments are present. Cores in this interval are poorly recovered and highly disturbed as a result of drilling. This unit probably represents a shallow-marine depositional environment. Unit VIUnit VI consists of gray, fine to medium, massive well-sorted sand. Rare shell fragments occur near the top and the bottom of this unit. The sand is quartz rich, and quartz grains are well rounded. Some decimeter-scale clay and silt-rich interbeds are recorded. At the bottom of the unit, pebbles and intraclasts are found. Based on the well-sorted nature of the sand, this unit may represent a high-energy fluvial or deltaic depositional environment. The mud interbeds may represent overbank deposits or channel fills. The rare shell fragments are likely locally reworked. Unit VIIUnit VII is dominated by a dark gray clast-poor sandy diamicton (Fig. F3) with dispersed (<1%) to uncommon (1%–5%) charcoal clasts up to 3 cm in diameter. The structure is mostly homogeneous with localized very rare silty to clayey laminae a few centimeters in thickness. Isolated intervals of dispersed (<1%) white carbonate rock fragments, fine mollusk shell hash, and silt intraclasts are present. The uppermost part consists of gray well-sorted clay/silt with locally clast-poor muddy to sandy diamicton. The clay appears mostly homogeneous with some weak lamination by color, especially in the upper part. Higher organic contents and strong odor were common. Fining upward of Unit VII between 158 and 146.1 mbsf was recorded. The base of this moderately sorted unit extends deeper than 229.6 mbsf, as it was not penetrated at the base of the hole. However, on open holing to 232.50 mbsf, the string became stuck and it was not possible to recover a sample to verify the lithology. Because of the general lack of visible grading and moderate sorting of Unit VII, deposition by mass-transport processes like massive debris flows is possible. The contacts between and thickness of individual debris flow beds are uncertain and potentially macroscopically not visible. The high charcoal content could be related to the outcropping of Jurassic sediments east of Site M0060. During the time of deposition, it is possible that large amounts of reworked Jurassic sediments including fossil soil horizons with coal seams were delivered to this location. The influence of mass-transport processes decreases to the top of this unit, indicated by increased sorting of clay/silt as a result of a change in the mode of sedimentation. |
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