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doi:10.2204/iodp.proc.347.107.2015 LithostratigraphyAt a water depth of 437 m, Site M0063 in the Landsort Deep is the deepest location drilled during Expedition 347, in the deepest basin of the Baltic Sea. Five holes were drilled: Hole M0063A to a total depth of 115.81 mbsf, Hole M0063B to 29.00 mbsf, Hole M0063C to 96.4 mbsf, Hole M0063D to 86.8 mbsf, and Hole M0063E to 92.8 mbsf. Piston coring was used in Holes M0063A–M0063E, except at the bottom of Holes M0063A and M0063C, where it became difficult because of the harder lithology of suspected diamicton. In Hole M0063A, an extended nose system was attempted, but recovery and penetration was low because of the highly variable lithology. In order to progress through the diamict, open-hole sections with spot sampling were carried out. At the base of the borehole, a hard layer was encountered, and hammer sampling recovered a granite rock ~5 cm in diameter (see “Operations”). During coring operations in Hole M0063A, problems were encountered because of the presence of methane gas–charged sediments. Pressure induced by a water depth of 437 m along with organic-rich sediments were responsible for extreme gas expansion, resulting in disturbed core sections in the form of sediment outburst and soupy intervals to frequent bubbles and cracks that were visible through the core liners to ~50 mbsf. The higher than expected expansion of the cores upon reaching the deck and the resultant loss of core material upon removal of the piston from Hole M0063A resulted in an alternative coring strategy. In the following holes, the piston coring system was pulled back 1.3 m from the base of each washed out interval before firing (see “Operations”). This procedure reduced the borehole advance to 2 m per core run but provided the most complete sequence within the expanding sediment package. Core recovery was exceedingly high, as much as 185% in the uppermost meters and ~150% on average at ~30 mbsf. At ~55 mbsf, core recovery returned to ~100%, and depth increments were increased to the normal 3.3 m run. The changes in procedure proved successful in Holes M0063B and M0063C and were therefore applied to Hole M0063D in order to secure a continuous stratigraphy and varve record. In the upper 20 m, methane gas escape remained a problem, resulting in moderately to highly disturbed sediments, and horizontal cracks were commonly found. The cored intervals were coordinated with earlier holes using stratigraphic correlation to obtain the most complete composite record of sediment with a good varve resolution ranging from the modern anoxic Landsort Deep through the Holocene and into late glacial clay. Hole M0063E was dedicated to microbiological sampling to ~90 mbsf. To avoid errors associated with expanding sediments, 2 m cores were taken to 54 mbsf. In doing so, it was assumed that the sediment would expand inside the core liner rather than beyond it, therefore reducing the amount of sediment lost and increasing availability for sampling purposes. Lithostratigraphic divisions at Site M0063 are based on descriptions of the cut face of the split core from Hole M0063D, as this is the most complete combined record of Holocene and late glacial sediments with an adjusted core recovery of nearly 99%. Supporting supplementary information is collected from the other Site M0063 holes, in particular from the deepest parts of Holes M0063A and M0063C. Information from smear slides from all holes is also included. Site M0063 is divided into seven lithostratigraphic units (Fig. F1). Unit I (0–25 mbsf) is composed of organic-rich diatom-bearing clays with subunits of weakly laminated intervals and prominently laminated intervals. Unit II (25–33 mbsf) is gray clay with an upper subunit dominated by iron sulfide lamination and a lower subunit of massive gray clay. A thin sand lamina indicates the shift to Unit III (33–41 mbsf), which comprises two subunits characterized by mixed clay sediments and contorted laminae. Unit IV (41–48 mbsf) is an iron sulfide–laminated gray clay subunit that gradually changes with depth to grayish brown varved clay. Unit V (48–53 mbsf) is contorted, convolute bedded clay. Unit VI (53–93 mbsf) is finely laminated varved clay with a downcore increase in the content of silt and sand demonstrated by thicker laminae and dispersed pebble content. The deepest part of the site is Unit VII (93–96 mbsf), which comprises sandy diamict. From 96 mbsf to the final depth of 115.8 mbsf (Hole M0063A), core recovery was very low. This, combined with water logging and the mixed character of the cores, made it impossible to resolve an interpretation of lithology. However, it is highly possible that the lithology represents a continuation of Unit VII to the bottom of Hole M0063A. Normal IODP procedure where expansion is concerned is to assign the top of a core to the depth of the base of the last core run and let the cores overlap. However, this is not useful when documenting lithostratigraphy. Therefore, in this section we have attempted to assign a linear compression factor to the expanded cores to avoid this overlap. Depths in the subsequent unit descriptions will therefore be given following the IODP noncompressed depths and the corrected compressed depths. These depths were recalculated using
Unit ISubunit Ia
Subunit Ib
Subunit Ic
Subunit Id
Unit I consists of organic-rich diatom-bearing clays comprising the uppermost 25.16 mbsf of Hole M0063D. An overall pattern of shifting weakly laminated (Subunits Ia and Ic) and prominently laminated (Subunits Ib and Id) intervals is visible. Unit I has a gradual lower boundary to less organic rich clays. Smear slide data (see “Core descriptions”) typically show very minor contents of silt and sand, abundant opaque authigenic minerals, and the existence of large centric diatoms. Subunit Ia consists of very well sorted black organic-rich clay with many soupy intervals and cracks throughout the core. Because of the high degree of gas expansion, no primary sedimentary structures are visible. Strong oxidation is found along the core liner and cracks, with a color change to olive-gray. In the best preserved intervals, black clay with a lamination color change at the millimeter scale and an organic algae mat-like appearance has been observed. Olive-colored oxidation is characteristic at the bottom of the section. Subunit Ic is similar to Subunit Ia, very well sorted black organic-rich clay only showing weak lamination and uncommon bioturbation by color at the millimeter scale. The subunit, to a varying degree, is oxidized to a greenish gray color. Subunit Id is the best preserved part of Unit I. The color ranges from full black to olive-gray and light yellowish brown. It is organic-rich clay with prominent laminations at the millimeter scale (Fig. F2). The sediment appears laminated, with single laminae of ~1–5 mm and a possible cyclicity at the centimeter scale. A peaty smell was noticed in the very well sorted sediment. The organic-rich clay with the shifting weak lamination and prominent algal mat-like lamination is possibly related to a shift between oxic and anoxic conditions in the marine Holocene sediments of the Baltic Sea (Zillén and Conley, 2010). Unit IISubunit IIa
Subunit IIb
In Unit II, the organic content is lower than in Unit I. The unit is divided into clay subunits because of a significant change from laminated to homogeneous sediment, a color shift from dark gray to gray, and intervals showing varying degrees of black sulfidic contents. Smear slide studies (see “Core descriptions”) in the uppermost part of Subunit IIa show very low contents of silt and sand, opaque authigenic minerals, and large centric diatoms, whereas Subunit IIb shows a high silt content of quartz and feldspar composition, with iron oxide coatings on quartz and large mica flakes. Subunit IIa is dark gray clay within which iron sulfide–stained, very dark gray weak lamination is visible at a millimeter–centimeter scale (Fig. F2). The clay is very well sorted. In Subunit IIb, the lamination disappears and the clay is homogeneous gray and well sorted. The lower boundary shows soft-sediment deformation toward a thin sand layer. In Unit II, sulfidization may be the result of breakdown of primary organic material in the laminations. The heavy disturbance, especially of Subunit IIb, however, cannot hide a clear change from a homogeneous Subunit IIb to laminated Subunit IIa clay, indicating a gradual change in depositional environment toward basin sedimentation. The stratigraphic position of Unit II just deeper than the organically rich clay and the iron sulfide lamination has on previous occasions been documented as Ancylus Lake sediments (Ignatius et al., 1981). Unit IIISubunit IIIa
Subunit IIIb
Unit III is composed of contorted clay deposits divided into two subunits. Both subunits show sharp lower boundaries and are upward bounded by centimeter-scale sand laminae. The top of Subunit IIIa is a heavily deformed, few centimeter thick, well-sorted, medium-grained sand layer. Below the sand follows massive, moderately sorted gray clay with contents of silt and sand, as well as a few dispersed pebbles of 0.5–1 cm size. This subunit is moderately disturbed by coring. As with Subunit IIIa, Subunit IIIb has an uppermost few centimeter thick, well-sorted, medium-grained sand layer followed by very well sorted clay with massive to convolute internal bedding structures and a sharp lower boundary. This subunit is slightly disturbed by coring. Smear slide data (see “Core descriptions”) show minor contents of silt, abundant contents of opaque authigenic minerals, and traces of possible altered volcanic glass with stretched vesicles. Subunit IIIa is classified as clay/silt with dispersed clasts, indicating slumping and mixing of sediments similar to Subunit IIIb and showing clear indications of displacements and contortion. The combination of contortion and thin sand layers can be interpreted as two slumping events coupled to basin currents. The presence of volcanic glass may be related to volcanic ash spreading events. Unit IVSubunit IVa
Subunit IVb
Unit IV is another laminated clay unit that gradually changes uphole from grayish brown lamination to gray with dark gray to black iron sulfide–stained lamination (Fig. F2). Subunit IVa is very well sorted greenish gray clay with dark gray weak iron sulfide lamination on a 0.5–1 cm scale. Subunit IVa gradually changes downcore to Subunit IVb. In Subunit IVb, very well sorted laminated clay continues, but the color changes to grayish brown and the laminae are thinner and recognized by millimeter- to centimeter-scale color banding. Smear slide data indicate nearly pure terrigenous clay. The gradual change from brownish lamination in Subunit IVb to iron sulfide lamination in Subunit IVa may indicate a shift in sedimentary environment away from glacial influenced lacustrine varvic sedimentation. Marine imprint on the youngest glaciolacustrine deposits is well known in the Baltic Sea history as the brackish phase of the Yoldia Sea stage deposits (Wastegård et al., 1995). Unit V
Unit V is dark gray to grayish brown very well sorted clay (Fig. F2). The internal structures show in part a massive appearance, but contorted silt laminated intervals are visible with clear convolute bedding. A smear slide (see “Core descriptions”), possibly from the silt laminae, shows 90% silt consisting of mainly quartz and feldspar, with iron oxide coatings on sand grains, green amphibole, glaucophane, and possible altered volcanic glass with stretched vesicles. The laminated clay indicates a possible glaciolacustrine depositional environment, whereas the convolute bedding shows that the Unit V clay deposits may have been remobilized in a slump event. As in Subunit IIIa, the existence of volcanic glass may be related to volcanic ash spreading events. Unit VI
Unit VI is the thickest unit at Site M0063. At more than 40 m thick, this unit shows significant development from the uppermost part where the unit is represented by dark gray–brown clay in millimeter- to centimeter-scale, interlaminated by millimeter-scale silt laminae (Fig. F2). With depth, several distinct varved fining-upward couplets are present, as well as sand laminae and pockets in millimeter scale. From the top of this unit to around 75 mbsf, scattered centimeter-scale carbonate concretions are identified within silt millimeter-scale laminae. The common shape of the concretions is elliptical (Fig. F3), and they are often found associated with a dropstone nucleus; multiple smaller concretions are observed on a larger dropstone. From ~85 mbsf, brown and weak red are the dominating colors and clay-silt-sand interlaminations are present in thicker units. Clay laminae 2–20 cm thick, occasionally with internal slumping and silt laminae on the millimeter scale, bundled to centimeter scale, are also common. In the lowermost part of Unit VI, well-sorted sand laminae and dispersed clasts occur. The lower boundary is gradational. Microscale transverse faults displace the laminated clay at the centimeter scale (Fig. F4). Smear slides (see “Core descriptions”) of clay and silty clay laminae show iron oxide coatings on quartz, light green amphibole, and possibly altered volcanic glass with stretched vesicles. Sand quartz grains are angular to subangular, and green amphibole is common. The rhythmically banded clays with varved internal grading are indicative of lake deposits, and the increased grain size and frequency of sand laminations in the lower sections, as well as subangular sand and centimeter-scale pebble grains, point to a glaciolacustrine environment. The upper part shows ice-distal facies types, whereas the lower ice-rafted part is deposited in an ice-proximal setting. As in Subunit IIIa and Unit V, the presence of altered volcanic glass in smear slides may be indicative of deposition of volcanic ash. Unit VII
The lowermost part of the core penetrates a brown, clast poor, sandy diamicton (Fig. F2) with a clast percentage of ~10% and a general pebble size of 0.2–0.5 cm. The maximum clast size is 4 cm. It is poorly sorted sediment, and the clasts are angular to subangular and faceted, consisting of rock clasts of quartz crystalline basement, Palaeozoic limestone, and sandstone. The poorly sorted sediment is also visible in smear slide samples (see “Core descriptions”) with high contents of clay, silt, and sand. Birefringent coatings on sand grains are common, and iron oxide coatings are also observed; sand grain shapes are angular–subangular. The diamicton of Unit VII can be interpreted as the result of mass-transport processes like massive debris flows or as a till with high content of locally eroded lithologies. |
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