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doi:10.2204/iodp.proc.303306.106.2006

Lithostratigraphy

Four holes were drilled at Site U1306 (Table T1). All cores were recovered using the APC. The sediments at Site U1306 are dominated by varying mixtures of terrigenous components and biogenic material (primarily quartz, detrital carbonate, and nannofossils) (see “Site U1305 smear slides” in “Core descriptions;” Fig. F5) so that the most common lithologies are silty clay, silty clay with diatoms, silty clay nannofossil ooze, and nannofossil silty clay. Burrowed and gradational contacts between these lithologies are much more common than well-defined or sharp boundaries. Abundances of terrigenous components, as estimated from smear slides, are quartz, 5%–75%; detrital carbonate, 0%–95%; feldspars, 0%–5%; clay minerals (including chlorite), 0%–70%; heavy minerals (especially hornblende), 0%–15%; and volcanic glass, 0%–5%. Smear slides taken from interval 303-U1306C-8H-1, 81–85 cm (71.12 meters composite depth [mcd]), and interval 303-U1306D-15H-4, 100–104 cm (146.83 mcd), have higher concentrations of volcanic ash. However, no distinct tephra layer was recognized in either case; instead, the higher abundance of volcanic ash appears to be present within burrow-filling sediment. Dropstones are common (at least one or more dropstones per core) throughout these cores (Fig. F6) and display a wide range of compositions, including acidic intrusive and metamorphic (granites, gneisses, and granitoids), basic igneous and/or metamorphic (basalts and metabasalts), and sedimentary and metasedimentary (sandstone and limestone) rocks. Abundances of biogenic components, as estimated from smear slides, are nannofossils, 0%–80%; foraminifers, 0%–55%; diatoms, 0%–40%; radiolarians, 0%–25%; and sponge spicules, 0%–8%. Total carbonate contents range from 0 to 12 wt% downhole (see “Geochemistry;” Table T24). Pyrite inclusions and burrows filled with pyritized sediment or surrounded by pyrite halos are abundant and constitute the only authigenic sediment components observed.

The sediments at Site U1306 are designated as a single lithostratigraphic unit, composed of Holocene–uppermost Pliocene (see “Biostratigraphy” and “Paleomagnetism”) terrigenous and biogenic sediments, which are gradationally interbedded at scales of a few meters or less.

Description of units

Unit I

• Intervals: Sections 303-U1306A-1H-1, 0 cm, to 33H-CC, 30 cm; 303-U1306B-1H-1, 0 cm, to 33H-CC, 22 cm; 303-U1306C-1H-1, 0 cm, to 28H-CC, 28 cm; and 303-U1306D-1H-1, 0 cm, to 19H-CC, 26 cm

• Depths: Hole U1306A: 0–307.0 mbsf, Hole U1306B: 0–315.9 mbsf, Hole U1306C: 0–267.6 mbsf, and Hole U1306D: 0–179.5 mbsf (0–333.47 mcd)

• Age: Holocene–Late Pliocene

Unit I is composed predominantly of silty clay and silty clay with diatoms. Nannofossil ooze with clay, silty clay nannofossil ooze, and silty clay with nannofossils; sandy clay, sandy clay with foraminifers, and clay are present as minor lithologies. Silty clay (with diatoms) is dark to very dark gray. Olive-brown sandy clay (with foraminifers), also defined as foraminifer ooze, is present in the uppermost 50 cm of Holes U1306A, U1306B, U1306C, and U1306D. This lithology is interpreted from smear slide data as the surface-oxidized equivalent of the underlying lithologies (Fig. F7). Other coarser-grained sediments are present as ~10 cm thick layers of foraminifer sand, clayey sand, and sandy clay. These lithologies are rarely present downcore and are gray in color. Olive-gray and/or greenish gray layers of clay or silty clay are present but rarely exceed 20 cm in thickness and are similar to the clay layers observed at Site U1305. These clay layers exhibit bioturbated boundaries and contain faint laminae of white silt. These greenish gray clay intervals are commonly burrow mottled, contain gravel or dropstones, and have a high detrital carbonate content (Figs. F8, F9).

All lithologies occur as horizontally bedded, mostly undisturbed sediments. The principal exception is drilling slurry, which typically occurs in the top section of each core and in core catchers. Disturbed intervals are rare and are observed only in intervals 303-U1306D-3H-5, 42–140 cm; 3H-7, 0–55 cm; 7H-3 and 7H-4, 0–150 cm; 7H-5, 55–98 cm; 7H-6, 0–150 cm; 7H-CC, 0–62 cm; and Core 10H. These intervals display deformed or flowed, sometimes faulted, structures. Some appear to be deformed mud clasts with variegated colors and irregular shapes (Figs. F10, F11). Some of these structures may be the result of deposition by debris flow or mass transport. However, coring disturbance is more likely because these features are present only in Hole U1306D, and the magnetic susceptibility (MS) data from Hole U1306D do not show any inconsistencies when compared to corresponding data from the three other holes. The sediment between the intervals specified above is undisturbed.

Dropstones are present in all four holes, and their distribution is plotted in Figure F6. Large dropstones (>2 cm) are common or abundant at this site (Fig. F12). Bioturbation is present throughout most of this unit; the most common indicators are diffuse centimeter-scale mottling and millimeter-scale burrows filled with pyrite concretions, white sand, foraminifer sand, or sponge spicules. In some cases, discrete burrows or macroscopic pyritized burrows were observed.

The silty clays at this site vary between dark gray (5Y 4/1) to very dark gray (5Y 3/1) and olive-gray (5Y 5/2). Intervals of silty clay with diatoms are very dark gray. Sandy clay and sandy clay with foraminifers (except for the surficial oxidized layers and foraminifer sand) are gray (5Y 5/1) to dark gray (5Y 4/1). Silty clay nannofossil ooze and nannofossil silty clay are gray, olive-gray, greenish gray (5GY 5/1), or dark greenish gray (5GY 4/1) and tend to have common to abundant bioturbation with pyritized burrows and dropstones. Contacts between these lithologies generally are gradational and commonly burrowed.

Discussion

The sediments at Site U1306 have biogenic and terrigenous components representing pelagic and hemipelagic deposition that have been modified by the interaction of bottom currents and topography on Eirik Drift. The resulting sediment composition reflects environmental changes that caused variations in phytoplankton and zooplankton assemblages and production and discharge of icebergs.

Large-scale patterns in the distribution of important lithologies at Site U1306 are presented in Figure F13. Of the 1070 m recovered, 1027 m (96%) is silty clay or silty clay with diatoms. Silty clay nannofossil ooze and nannofossil silty clay compose 21.5 m (2%) of the sediments, whereas nannofossil ooze composes 11 m (1%) of the sediments. The remaining 10.5 m (1%) consists of sandy clay, clayey sand, and foraminifer sand. The relative abundances of the major lithologies is remarkably consistent downcore.

Centimeter- to decimeter-scale beds of olive-gray (5Y 5/2) or (dark) greenish gray (5GY 4/1 and 5GY 5/1) clays or silty clays with high detrital carbonate are similar in appearance to detrital carbonate clay intervals identified at Site U1305 but are less common and thinner at Site U1306. However, the relationship between these olive/greenish gray detrital carbonate beds and the MS record is not straightforward.

Smear slides were taken at least once every two cores to document the composition of the dark gray silty clay that dominates these sediments. The quartz content of the sediment is less variable downcore than other components (Fig. F14). In contrast, both the detrital carbonate and the nannofossil contents are unusually low from 0 to 50 and from 150 to 250 mcd, respectively (except for a peak at 194 mcd). The foraminiferal abundance also decreases from abundant (~40%) to poor or barren below 150 mcd (see “Biostratigraphy”). These low-carbonate intervals may be explained by dilution, dissolution, or low input. The preservation index for the coccolithophorids and planktonic foraminifers is moderate to good in these intervals, suggesting that dissolution is not a factor. The biostratigraphic and paleomagnetic age models do not show any major changes in sedimentation rates.

Gravel counts, defined as the number of clasts >2 mm in each 10 cm length of core, were made in all four holes, yielding a total of 632 clasts or 1.7 clast every 3 m of core. These abundances are too low to make meaningful interpretations about downcore variations. However, it is clear that the dropstones are present more frequently at Site U1306 than Site U1305 (~1 dropstone/3 m of core). Deposition at these two sites is characterized by expanded interglacials at Site U1305, whereas Site U1306 has thicker glacial intervals. This model could explain the higher dropstone frequency at Site U1306. The granitic and basaltic dropstones are consistent with known source areas in east Greenland.

The lightness (L*) of the color reflectance was measured every 2 cm in each undisturbed core section. L* ranges between 25% and 50% at Site U1306; whereas L* varies from 28% to 53% at Site U1305. This difference is ascribed to a lower average CaCO₃ content at Site U1306 (3.2 wt%) versus Site U1305 (12.3 wt%) (see “Geochemistry”). For the interval between 40 and 150 mcd at Site U1306, L* and MS show a strong inverse relationship such that higher CaCO₃ imparts a light color and low MS to the sediments (Fig. F15). This relationship is less clear for the intervals 0–40 mcd and below 150 mcd. CaCO₃ content and abundances of calcareous nannofossils and planktonic foraminifers show significant decreases below 150 mcd. For the intervals between 0 and 40 mcd and below 150 mcd, L* and MS may respond to the abundance of siliciclastic grains (Fig. F15) (Balsam et al., 1999).

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