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doi:10.2204/iodp.proc.341.106.2014 LithostratigraphyA lithologic summary of Site U1420 is shown in Figure F5. The total depth of Hole U1420A was 1014.5 m core depth below seafloor (CSF-A). Recovery from 58.2 to 448.5 m CSF-A was <10% but improved deeper than 448.5 m CSF-A, where several cores were collected with recovery between 30% and 94%. Total core recovery for the entire site was 14%. Additionally, numerous drilled rocks and washed clasts were recovered without a supporting matrix lithology. Cores containing mostly drilled rocks and washed clasts include 341-U1420A-10R through 47R, 49R, 52R through 57R, 75R through 79R, and 85R. The limited sediment recovered at Site U1420 contains seven 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 (F4f and F4e) are very dark gray (N 3) to dark gray (N 4) clast-rich and clast-poor diamict. Photographs of the most common facies are shown in Figure F6. Based on characteristic facies associations, three lithostratigraphic units were defined (Table T3). Facies descriptionSeven lithofacies were identified and are outlined in Table T2. Most 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 U1420 are described here. These include massive mud with lonestones (F1a), massive mud without lonestones (F1b), clast-poor diamict (F4e), clast-rich diamict (F4f), mud with diatoms/biosilica (F5b), calcareous/carbonate-bearing mud (F5c), and volcaniclastic mud and diamict (F7). The massive and bioturbated mud with lonestones of Facies F1a is mostly dark gray (N 4) to dark greenish gray (10Y 4/1) and has bed thicknesses that range from 3 to 300 cm (Table T2; Fig. F6A). Clast abundance within the mud ranges from dispersed to abundant. Bioturbation is mostly absent but occasionally slight to heavy. A diagnostic characteristic of Facies F1a is the absence or low abundance of microfossils. Lonestones consist mainly of siltstone, sandstone, and basalt (Fig. F7A–F7G). Occasional foraminifers were observed in smear slides. Facies F1b is composed of dark greenish gray (5GY 4/1 or 10Y 4/1) to very dark gray (N 3) mud, comprising 3 to 409 cm thick intervals (Table T2; Fig. F6B). Bioturbation is mostly moderate to heavy, and shell fragments are present. Diatoms and foraminifers are found rarely. Facies F4e is very dark gray (N 3) to dark gray (N 4) clast-poor diamict, most often with a silty or sandy mud matrix (Fig. F6D). Beds are massive, and thickness ranges from 11 to 275 cm. Common clast sizes are granule and pebble, with clasts being subangular to subrounded. Dominant clast lithologies are siltstone, sandstone, basalt, granitoids, argillite, rhyolite, and metasandstone. Bioturbation is absent, but some shell fragments are present. In more muddy intervals, very thin mud beds were found occasionally, as well as mud clasts and isolated sand laminae. Facies F4f is very dark gray (N 3) clast-rich diamict, typically with a muddy matrix and occasionally containing shell fragments (Table T2; Figs. F6D–F6E, F7H–F7I). Bed thickness ranges from 2 to 556 cm. Common clast sizes and lithologies are similar to those described for Facies F4e. Two intervals with visually apparent clast orientation were documented in Cores 341-U1420A-92R and 93R (Fig. F7A). Facies F5b consists of intervals 42 to 140 cm thick of dark greenish gray (10Y 4/1) mud with dispersed clasts containing small amounts of diatoms and sponge spicules (Table T2; Fig. F6F). Shell fragments were found occasionally, and traces of volcanic ash occur very rarely. Bioturbation is either absent or heavy. Facies F5c contains very dark gray (N 3) to dark greenish gray (5GY 4/1) mud and diamict with foraminifers and rare gastropods (Table T2; Fig. F6G). This facies is 42 to 281 cm thick. Bioturbation varies from absent to heavy. Very dark gray (N 3) to dark greenish gray (10Y 4/1) diamict and mud with traces of volcanic ash define Facies F7 (Table T2), and intervals commonly range in thickness from 18 to 140 cm. Lithostratigraphic unitsBased on facies associations, three lithostratigraphic units were defined (Table T3) for Site U1420. The contacts between lithostratigraphic units at Site U1420 are usually not observed because of the poor core recovery. The criteria used to define units are discussed below. Unit I
Lithostratigraphic Unit I consists of very dark gray (N 3) muddy clast-rich diamict interbedded with clast-poor diamict with angular to rounded clasts. Clast size ranges from granule to pebble. The diamict beds are massive and have mainly a mud matrix with some outsized sand grains, based on smear slide observations. A few beds have a sandy matrix. Unit II
Recovery within Unit II ranged from 0% to 8%. The major lithology was not recovered, and cores mostly contain washed pebbles and cylindrical shaped drilled rocks with abraded surfaces (Fig. F7C–F7G). The cylindrical shape suggests that the minimum diameter of drilled cobbles or boulders exceeded the inner diameter of the drill bit (6.20 cm). Small amounts of sediment were occasionally recovered (rarely exceeding 10 cm and usually heavily disturbed). Unit III
Lithostratigraphic Unit III consists of very dark gray (N 3) clast-rich diamict and very dark gray (N 3) to dark gray (N 4) clast-poor diamict interbedded with very dark gray (N 3) to dark greenish gray (10Y 4/1) mud with or without lonestones. Some diamict intervals contain shell fragments. Diamict intervals dominate the unit, but intervals of mud occur in Cores 341-U1420A-51R through 52R, 59R, 60R, 63R, 68R, 80R, 83R, 88R, 90R through 93R, and 99R. The amount of lonestones in massive mud intervals ranges from dispersed to abundant. However, mud without clasts occurs in Cores 80R and 83R. Bioturbation is mostly absent but is moderate to heavy in mud without lonestones. Drilled rocks occur in Cores 75R through 79R and 85R, coincident with low core recovery. Two subtle erosional surfaces are observed in Cores 92R and 93R in proximity to clasts that are oriented parallel to the bedding plane (Fig. F7A). Additional intercalated very dark gray (N 3) and dark gray (N 4) clast-rich diamict is found in Core 98R (Fig. F6H). PetrographyClast lithologiesThe main lithologies of the drilled rocks, washed pebbles, clasts within the diamict, and lonestones contained in the sediment (Fig. F7A–F7G) are, in order of decreasing abundance: sandstone, siltstone, basalt, and granitoids. The granitoid group includes intermediate and felsic intrusive rocks. Argillite, rhyolite, and metasandstone represent minor lithologies. The average clast abundance for Site U1420, according to the main lithology types, metamorphic (M), igneous (I), and sedimentary (S), is M8I35S57 (Fig. F8). One clast contained thin veins of copper. Bulk mineralogyX-ray diffraction (XRD) analyses were performed on 39 powdered bulk samples from Hole U1420A to delineate the bulk mineralogy and identify compositional trends with age or depth in the cores. The resulting diffraction patterns are shown in Figure F9, 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. 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 F9B 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]). This treatment was used to determine the presence of expandable clay minerals (e.g., smectite). The scans generally suggest the presence of expandable clay minerals from ~468 m CSF-A downhole. Our preliminary findings are similar to the results of Molnia and Hein (1982) from samples collected on the continental shelf of the Gulf of Alaska. Lithostratigraphy and depositional interpretationsThe distribution of primary sedimentary lithologies and core recovery at Site U1420 is summarized in Fig. F10. With the exception of three intervals of clast-rich and clast-poor diamict between 10 and 50 m CSF-A, core recovery was <10% in Units I and II (uppermost 450 m CSF-A) and largely consisted of drilled rocks and washed pebbles. Core recovery in Unit III increased above 10% between 450 and 500, 540 and 670, and 750 and 980 m CSF-A. Recovered sediment from these intervals consists of clast-rich and clast-poor diamict, and extensively bioturbated (bioturbation intensity index = 3) dark greenish gray (10Y 4/1) mud with or without clasts is found between 595 and 597 m CSF-A and between 760 and 780 m CSF-A. Mud with abundant and common clasts is observed between 860 and 880 m CSF-A. Two ~50 m thick intervals with low core recovery (<10%) are found centered at 525 and 725 m CSF-A and largely consist of drilled rocks and washed pebbles. Unit IA survey by drill string–mounted camera prior to drilling at Site U1420 revealed smooth seafloor covered by fine-grained sediment with a lone boulder. This part of the depositional sequence was not recovered by drilling (Fig. F10). The top of the recovered record (beginning in Section 341-U1420A-2R-1) contains a massive clast-rich diamict, which is delineated as lithostratigraphic Unit I. We suggest that this diamict was deposited from intense iceberg rafting during glacial times, when icebergs transported and deposited large quantities of debris, along with mud from meltwater plumes. Our suggestion is based primarily on the absence of oriented clasts that frequently occur in subglacial deposits (Benn and Evans, 2010). An alternative interpretation is that the diamict in Unit I could have been deposited subglacially. This interpretation is based on the stratigraphic position of the upper part of Unit I, located shallower than the regional unconformity shown on the geophysical lines through Site U1420 (Figs. F11). This relationship implies that grounded ice may have overridden Site U1420 during advance(s) across the shelf. Both tentative interpretations for Unit I require additional sediment fabric analyses for verification. Unit IIMainly washed pebbles and drilled rocks (Fig. F7C–F7G) with limited fine-grained lithologies were recovered in lithostratigraphic Unit II. It is impossible to interpret the depositional environment of these isolated clasts without the context provided by the matrix and other “missing” fine-grained lithologies. Owing to this limitation, lithofacies from this unit were not included in the lithofacies table (Table T2). Assuming that the diameter of the drilled rock pieces is proportional to the drilled length (up to 30 cm) (Fig. F7G), some of the clasts in Unit II are boulder size. This indicates that a significant number of subsurface boulders likely influence drilling and the success of recovery in some intervals (Fig. F10). Concentrations of boulders in the upper portion of Unit II (located shallower than the regional unconformity) (Fig. F6) may be boulder pavements, potentially formed by lodgment processes beneath grounded glaciers (Powell and Cooper, 2002). Alternatively, they may be similar to the boulder pavements described in the glacimarine Yakataga Formation from Middleton Island, Gulf of Alaska, where they form planar surfaces that can be traced several kilometers along strike (Eyles and Lagoe, 1990; Eyles, 1987). The sources for clasts documented in Unit II are interpreted to be the onshore St. Elias and Chugach Mountains located along the southern coast of Alaska. Unmetamorphosed siltstone and sandstone clasts, the most common clast types (Fig. F8), may have been derived from the Kulthieth, Poul Creek, and Yakataga Formations. These Cenozoic strata are exposed in the active onshore thrust belt (Plafker, 1987; Pavlis et al., 2012) and are currently being deeply eroded by glaciers. The less common metasedimentary clast lithologies were probably derived from the Mesozoic accretionary prism strata and metamorphic core complex exposed in the Chugach and St. Elias Mountains (Plafker et al., 1994; Gasser et al., 2011). Felsic igneous clasts may have derived from the Sanak-Baranoff plutons found dispersed along the southern Alaska margin (Sisson et al., 2003). Unit IIIThe interbedded diamict and mud in Unit III is interpreted to represent fluctuations in the grounding-line position and variations in the supply of ice-rafted debris and meltwater in a proglacial setting. The dominance of diamict and mud with lonestones suggests the continuous presence of tidewater glaciers on the continental shelf during the deposition of Unit III. The two intervals with poor recovery (<10%) and abundant drilled rock in Unit III are suggested to represent periods of enhanced ice rafting. Intervals of massive mud without lonestones and moderate to heavy bioturbation (Facies F1b) were possibly deposited during periods of reduced glacial influence. Diamict intervals with intercalated structures, crude horizontal fabric, and beds that grade upward into mud in the lower parts of Unit III are interpreted to be sediment gravity flow deposits. This interpretation is supported by biological evidence, including shell fragments and the occurrence of shallow-water foraminifers (see “Paleontology and biostratigraphy”). The facies associations described above are similar in many respects to a stratigraphic model for glacial cycling on the Alaskan shelf proposed by Powell and Cooper (2002). Mud without lonestones facies correlate with their “Glacial Minimum Systems Tract,” which represents minimum glacial influence. Mud containing lonestones (ranging in abundance) and diamict would have been deposited in distal to proximal glacimarine environments that correlate to the “Glacial Advance Systems Tract” in this model. The boulder accumulations in the poor-recovery intervals are suggestive of the most glacier-proximal conditions within the sequence (Fig. F7). In general, some of the recovered sediment at Site U1420 may include representative sections of glacial sequences as described in the model by Powell and Cooper (2002). We suggest that differences between our observations and the model can be explained by the limited core recovery or local differences in sediment supply and glacial conditions. |
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