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doi:10.2204/iodp.proc.341.107.2014

Lithostratigraphy

Lithologic summaries of the three holes drilled at Site U1421 are shown in Figure F5. The sediment recovered at Site U1421 contains nine facies. Detailed facies descriptions, information about common marine microfossils, facies occurrence in lithostratigraphic units, and tentative interpretations about depositional environments are summarized in Table T2. The dominant facies (F1a, F1b, F4e, and F4f) are dark gray (N 4) to dark greenish gray (10Y 4/1) mud and diamict. They account for >95% of the cores recovered. Photographs of the most common facies are shown in Figure F6. Based on characteristic facies associations, two lithostratigraphic units were defined (Table T3).

Facies description

Nine lithofacies were identified and are outlined in Table T2. All of these are included within the group of facies observed at other Expedition 341 sites. Facies numbering is based on those documented for all sites, but only facies documented at Site U1421 are discussed here. These include massive mud with lonestones (F1a), massive mud without lonestones (F1b), interbedded mud and diamict (F4d), clast-poor diamict (F4e), clast-rich diamict (F4f), diatom ooze (F5a), mud with diatoms/biosilica (F5b), calcareous/carbonate-bearing mud (F5c), and mud and diamict with volcanic ash (F7).

The massive mud with lonestones of Facies F1a is mostly very dark gray (N 3) to dark greenish gray (10Y 4/1) and has bed thicknesses that range from 149 to 992 cm (Table T2; Fig. F6C). Clast abundance within the mud ranges from dispersed to abundant. Bioturbation is mostly absent. However, one interval is heavily bioturbated and contains traces of ash. Lonestones consist mainly of siltstone, sandstone, and basalt (Figs. F7A–F7I). This facies can occasionally be well consolidated (e.g., Sections 341-U1421A-7H-5 through 8H-3), and lamination occurs rarely. Facies F1b is composed of dark greenish gray (10Y 4/1) to dark gray (N 4) mud, comprising 5 to 595 cm thick intervals (Table T2; Fig. F6A–F6B). Bioturbation is mostly absent but is slight in one interval. Diatoms are relatively abundant, and shell fragments, foraminifers, and black mottles occur occasionally. Rare laminations are observed.

Dark gray (N 4) clast-poor diamict interbedded with laminae and up to 4 cm thick beds of mud composes Facies F4d (Table T2). The diamict contains granule and pebble clasts of siltstone, sandstone, granitoids, and greenstone. Bioturbation is absent, and shells are rare. Facies F4e is very dark gray (N 3) clast-poor diamict, most often with a silty or muddy matrix (Table T2; Fig. F6F). Beds are massive, and thicknesses range from 13 to 465 cm. Common clast sizes are granule and pebble, with clasts being subangular to subrounded. Dominant clast lithologies are sandstone, siltstone, basalt, argillite, granitoid, rhyolite, and metasandstone (Fig. F7A–F7I). Bioturbation is absent, but some shell fragments are present. Facies F4f is very dark gray (N 3) to dark gray (N 4) clast-rich diamict, typically with a muddy or silty matrix, often containing sand (Table T2; Fig. F6G). Shell fragments occur rarely, and bioturbation is absent. Bed thicknesses range from 3 to 344 cm. Common clast sizes and lithologies are similar to those described for Facies F4e.

Facies F5a is composed of dark greenish gray (10Y 4/1) diatom ooze. It includes 12 to 142 cm thick intervals that are either laminated without bioturbation or that are massive and slightly or heavily bioturbated (Table T2; Figs. F6D, F7J). Lonestones are absent, and shell fragments occur occasionally (Fig. F7K). Facies F5b consists of 19 to 595 cm thick dark greenish gray (10Y 4/1) mud with absent to abundant clasts. This facies is most often diatom bearing and contains occasional sponge spicules (Table T2; Fig. F6H). Bioturbation is mostly absent or limited. Shell fragments, black mottles, and foraminifers occur occasionally. Trace amounts of volcanic ash are rare. Facies F5c contains grayish green (10Y 4/2) diatom-rich mud with foraminifers (Table T2). This facies is found in 15 to 18 cm thick intervals. Bioturbation is slight, and black mottles are present. Facies F7 is defined as dark gray (N 4) diamict and dark greenish gray (10Y 4/1) diatom-bearing mud with traces of volcanic ash (Table T2). Bioturbation is heavy in the mud but absent in the diamict. The thickness of this facies varies from 46 to 67 cm.

Lithostratigraphic units

Based on facies associations observed in Holes U1421A–U1421C, two lithostratigraphic units are defined (Table T3). The contact between lithostratigraphic units at Site U1421 is gradational, and the criteria used to define units are discussed below.

Unit I

• Intervals: 341-U1421A-1H-1, 0 cm, to 7H-5, 20 cm; 341-U1421B-1H-1, 0 cm, to 1H-CC, 18 cm; 341-U1421C-1H-1, 0 cm, to 6H-CC, 36 cm
• Depths: Hole U1421A = 0–57 m core depth below seafloor (CSF-A); Hole U1421B = 0–6.23 m CSF-A; Hole U1421C = 0–38.22 m CSF-A
• Age: Middle Pleistocene to Holocene

Very dark gray (N 3) to dark greenish gray (10Y 4/1) mud, diatom-bearing mud, and diatom-rich mud extends from 0 to 57 m CSF-A in Hole U1421A. The amount of lonestones varies from dispersed to abundant deeper than 6.4 m CSF-A. One interval of olive (5Y 5/3) laminated mud was deposited above the bed containing the uppermost lonestones (Fig. F6B). All recovery in Holes U1421B and U1421C is in Unit I.

Unit II

• Interval: 341-U1421A-7H-5, 20 cm, to 85X-1, 95 cm
• Depth: Hole U1421A = 57–702.7 m CSF-A
• Age: Middle Pleistocene to Holocene

Dark gray (N 4) to very dark gray (N 3) clast-poor and clast-rich diamict, as well as mud with abundant clasts, characterize this unit. Diamict beds contain subrounded to subangular, granule- to pebble-sized clasts. The clasts have diverse lithologies, including siltstone, sandstone, argillite, quartz, greenstone, granitoid, and diorite. The Unit I/II boundary corresponds with an increase in gamma ray attenuation (GRA) bulk density from 1.9 to 2.2 g/cm³, where it remains throughout the clast-rich intervals within Unit II (Fig. F8). Biosilica-rich or diatom-rich mud occurs in Cores 341-U1421A-22X, 41X, and 55X through 57X. Diatom ooze occurs in Cores 61X through 63X and 75X. Dark greenish gray (10Y 4/1) finely laminated diatom ooze occurs in Section 341-U1421A-63X-1.

Petrography

Clast lithologies

The main lithologies of the diamict clasts and lonestones contained in Site U1421 sediment (Fig. F7A–F7I) are, in order of decreasing abundance, sandstone, siltstone, basalt, argillite, granitoid, and rhyolite. The granitoid group includes intermediate and felsic intrusive rocks. Metasandstone, gneiss, metasiltstone, mudstone, and greywacke represent minor lithologies. The distribution of lithologies, according to the main lithology types, metamorphic (M), igneous (I), and sedimentary (S) (Fig. F9), reveals the predominance of sedimentary lithologies over igneous and metamorphic ones. The average clast ratio for Site U1421 is M₁₂I₃₃S₅₅ (Fig. F9).

Bulk mineralogy

X-ray diffraction (XRD) analyses were performed on 29 powdered bulk samples from Hole U1421A to characterize the bulk mineralogy and identify compositional trends with age or depth in the cores. The resulting diffraction patterns are shown in Figure F10, and the relative mineral diffraction peak intensities, as defined in “Lithostratigraphy” in the “Methods” chapter (Jaeger et al., 2014), are listed in Table T4. In general, the mineralogy was uniform downhole, although there are some variations in relative peak intensities, which may indicate slight variations in mineral content. Figure F10A shows the scans for five representative samples. The primary minerals identified include quartz, plagioclase (feldspar), mica (muscovite/illite and biotite), and chlorite and/or kaolinite. Quartz and plagioclase are the dominant peaks, with quartz generally the larger, and chlorite and/or kaolinite are present in all samples. Figure F10B shows the comparative XRD patterns from 4° to 24°2θ, where the scans were run before and after the samples had undergone a glycolization treatment (see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014]). The scans suggest the presence of expandable clay minerals from ~95 m CSF-A downhole, with the exception of the sample at ~295 m CSF-A. Our preliminary findings are similar to the results of Molnia and Hein (1982), which analyzed samples collected on the continental shelf of the Gulf of Alaska.

Lithostratigraphy and depositional interpretations

The distribution of primary sedimentary lithologies at Site U1421 is summarized in Figure F11. Unit I is characterized by the presence of very dark gray (N 3) to dark greenish gray (10Y 4/1) mud with varying clast concentrations (dispersed, common, and abundant). Dark greenish gray (10Y 4/1) diatom-rich mud and diatom-bearing mud is dominant in the upper 8.3 m and exhibits millimeter-scale laminations over a 10 cm thick interval centered at 6.1 m CSF-A. Unit II mostly consists of clast-rich and clast-poor diamict, with minor amounts of diatom ooze and diatom-rich mud. Bioturbation is heavy (intensity index = 3) in some but not all of these biosiliceous intervals; laminated diatom-rich mud and diatom ooze are observed between 441 and 442 m CSF-A and 494 and 500 m CSF-A, respectively. Core recovery averages 24% within Unit II, and low-recovery cores (<10%) contain washed pebbles, drilled rock, and <0.5 cm thick intervals of clast-rich diamict. The low recovery interval between 25 and 36 m CSF-A was partly recovered in Hole U1421C (see “Stratigraphic correlation”) and largely consists of mud with abundant and common clasts, which is the same lithology observed on Hole U1421A. Sediment cores from Hole U1421C contain mud with common and abundant clasts.

Unit I

Lithostratigraphic Unit I consists mainly of diatom-bearing and diatom-rich mud with lonestones and silty mud with dispersed to abundant clasts. Lonestones occur deeper than 6.4 m CSF-A in Hole U1421A. We interpret most of the mud as having been deposited through suspension settling from turbid meltwater plumes from tidewater glaciers. The diatom-rich intervals might be related to one or several processes:

• Increased biological productivity due to optimized oceanographic conditions (e.g., reduced sea ice cover, surface layer overturning, and/or mixing by gyres; Addison et al., 2012);
• Enhanced macro (N, P) and/or micro (Fe) nutrient supply from land (through volcanic ash, dust, etc.) leading to increased biological productivity (Addison et al., 2012; Davies et al., 2011; Hamme et al., 2010);
• Increased biological productivity in the water column in the vicinity of sea ice margins (Sakshaug, 2004; Smith et al., 1987);
• Seawater silica saturation, leading to a higher diatom productivity and preservation (e.g., Brzezinski et al., 1998; Dugdale et al., 1995); and/or
• Decreased input of terrigenous sediment (i.e., less dilution).

Thin laminations (Fig. F6B) observed in the diatom-bearing mud shallower than 6.4 m CSF-A in Hole U1421A (and also in the upper 8 m CSF-A at Site U1419) may point to hypoxic benthic conditions associated with enhanced sea-surface productivity as previously documented by Addison et al. (2012).

The lonestones are interpreted to have been rafted by icebergs calved from tidewater glaciers (Davies et al., 2011; Powell and Molnia, 1989). The sources for clasts documented in Unit I are interpreted to be the onshore St. Elias and Chugach Mountains located along the southern coast of Alaska. Siltstone and sandstone clasts, the most common clast types (Fig. F9), may have been derived from the Kulthieth, Poul Creek, and Yakataga Formations. The large number of volcanic clasts, especially the basaltic clasts, may have derived from the basaltic strata in the Poul Creek Formation (Plafker, 1987). The less common metasedimentary clast lithologies were probably derived from the Mesozoic accretionary prism strata and the metamorphic core complex rocks exposed in the Chugach and St. Elias Mountains (Plafker et al., 1994; Gasser et al., 2011). Felsic igneous clasts may have been derived from the Sanak-Baranoff plutons found dispersed along the southern Alaska margin (Sisson et al., 2003).

Unit II

Clast-rich and clast-poor diamict (Facies F4e and F4f) are the major lithologies of Unit II, suggesting a glacigenic origin for these deposits. The sedimentary processes leading to the deposition of the diamict facies include ice rafting and downslope transport of glacigenic sediment deposited at or beyond the grounding line in the vicinity of the shelf break.

Overconsolidation measured with the vane shear in Sections 341-U1421A-7H-5 through 8H-3 (Fig. F12A) suggests either deposition by a glacigenic debris flow or downslope transport of a coherent block of subglacial sediment (e.g., Vorren et al., 1998). Other evidence of downslope transport includes erosive lower boundaries of diamict and soft-sediment deformation in intervals of laminated diatom ooze within diamict (e.g., Core 341-U1421A-62X; Fig. F6J).

The main source of the reworked sediment is most likely the Bering Trough, located upslope from Site U1421, which may have acted as a pathway for an ice stream draining parts of the Cordilleran Ice Sheet into the Gulf of Alaska and transporting large amounts of sediment to the shelf break. Muddy intervals with biogenic silica and low clast abundance are suggested to reflect reduced ice rafting and/or increased productivity.

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