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doi:10.2204/iodp.proc.341.103.2014 LithostratigraphyLithologic summaries of the five holes drilled at Site U1417 are shown in Figure F10. The sediment recovered at Site U1417 contains 17 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 and F1b) are gray (N 4) to dark greenish gray (10Y 4/1) mud and account for >90% of the core recovered. The remaining minor facies, although much less volumetrically significant, are distinctive and allow us to organize the cores into lithostratigraphic units. Photographs of the more common facies are shown in Figure F11. Based on characteristic facies associations, 5 major lithostratigraphic units and 12 subunits are defined (Table T3). Facies descriptionSeventeen lithofacies were identified and are outlined in Table T2. The description of the facies for this site is based on the general lithofacies developed for all Expedition 341 sites. Lithofacies described at this site include massive mud with lonestones (F1a); massive mud without lonestones (F1b); silt (F2a); interbedded silt and mud (F2b); very fine to coarse sand (F3a); medium to coarse sand (F3b); interbedded sand and mud (F3c); muddy diamict (F4a); muddy and sandy diamict with lithic and mud clasts and/or terrigenous organic components (F4b); breccia (F4c); interbedded mud and diamict (F4d); diatom ooze (F5a); biosiliceous ooze (F5b); calcareous/carbonate-bearing/rich mud, silt, sand, diamict, and/or diatom ooze (F5c); volcanic ash (F6); volcaniclastic mud and sand (F7); and rock (F8). These facies reflect deposition from suspension fall out, sediment gravity flows, ice rafting, variations in organic productivity, volcanic eruptions, and eolian processes. The lonestone-bearing, massive and bioturbated mud of Facies F1a is dark gray (N 4) to dark greenish gray (10Y 4/1) (Fig. F11A–F11B; Table T2). Color banding in this facies ranges from gray (N 4) to dark greenish gray (10Y 4/1), and individual bands range in thickness from 0.1 to 5 cm. Bioturbation ranges from moderate to complete (for bioturbation scale, see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014]). Based on smear slides, the composition of the mud is on average 90% clay size particles and 10% silt particles, with no apparent compositional differences between different color bands. Microfossil content is variable. Biosiliceous components are predominantly diatom microfossils, but minor amounts of sponge spicules and nannofossils are also documented. Lonestone clasts consist mainly of argillite and metasiltstone with secondary amounts of metabasalt and minor amounts of granite and sandstone (Fig. F12A–F12D). Facies F1b (Table T2) consists of greenish gray (10Y 5/1) to dark gray (N 4) mud without lonestones. Microfossil content is variable, as in Facies F1a. Zoophycos burrows are common in this facies, and relative to Facies F1a, color banding is not common. Petrographically, the composition of Facies F1b is similar to that of Facies F1a. In a minor number of beds in Facies F1b, diatoms, sponge spicules, and burrows have been pyritized (Fig. F12K). Facies F2a and F3a (Table T2) consist of very thin to thin silt and sand beds (Fig. F11B) and are discussed together because of their common occurrence in the cores from Site U1417. These two facies range in color from dark gray (N 4) to greenish gray (5GY 5/1) and are mostly interbedded with the mud of Facies F1a and F1b. The silt and sand beds have sharp and sometimes erosional lower contacts with the underlying mud and primarily gradational upper contacts (Fig. F11B). Load casts occur along the bases of some beds in this facies (Fig. F12E). Individual bed thicknesses are 1–5 cm, and normal grading is common. These facies are often well sorted and contain little fine-grained matrix. The grain composition of these two facies is almost entirely quartz and feldspar, with minor amounts of heavy minerals. Quartz in these facies is monocrystalline, and few lithic grains are documented. Less common are silt and sand beds rich in mafic components such as hornblende, biotite, and opaque minerals. The more mafic silt and sand beds are also enriched in heavy minerals relative to the more quartzofeldspathic sand and silt in this facies. In general, quartzofeldspathic sand and silt comprise ~85% of the total sand and silt in these facies. Facies F2b consists of dark gray (N 4) to very dark gray (N 3) interbedded silt and mud. Lower contacts of silt laminae are most often sharp. Upper contacts are sharp or gradational. Bioturbation is none to slight. Facies F3b (Table T2) consists of dark gray (N 4) to gray (N 5) medium to coarse sand and is distinguished from Facies F3a by its accessory components (Fig. F11E). This facies often has a sharp lower contact and a gradational upper contact. Bed thickness is up to 40 cm, but average bed thickness is ~10–20 cm. The sand has a very muddy matrix, and intraformational rip-up clasts are common (Fig. F11F). Coal clasts and woody detritus also occur in this facies (Figs. F11E, F12G–F12J). Based on smear slides, the composition of sand in Facies F3b is lithic rich with sedimentary (mainly mudstone and siltstone), volcanic, and metasedimentary (mainly argillite) grains. These sand beds also contain more mica than other sand facies at Site U1417. In smear slides, most mica appears to be biotite, but lighter colored micas are also identified in the visual core descriptions. Quartz in this facies is mainly monocrystalline, but polycrystalline quartz grains are also identified. Heavy minerals are a common minor constituent of Facies F3b. Facies F3b is gradational with the sandy diamict of Facies F4b. Facies F3c consists of interbedded sand and mud (Table T2). This facies is between 7 and 710 cm thick. Graded sand beds have sharp lower boundaries and sharp upper boundaries. Bioturbation is mostly absent in this facies. Muddy diamict in Facies F4a (Table T2) is characterized by a dark gray (N 4) mud-rich matrix. Distinctive features of this facies are gradational lower contacts and sharp upper contacts defined by the relative clast concentration (Fig. F11C). Common clast sizes are granule and pebble, with clasts being subangular to subrounded. Based on smear slides, the composition of the sand fraction is primarily quartzofeldspathic. Facies F4b consists of muddy and sandy diamict with bed thickness up to 40 cm and containing predominantly mud clasts and/or organic components (Table T2; Fig. F11D). A sand and mud matrix with large outsized intraformational clasts characterizes this facies. Mud clasts can reach 3 cm in diameter. Other common clast types are coal and woody debris. These beds have sharp upper and lower contacts. Petrographically, this facies is characterized by more lithic grains in the sand fraction, similar to those described for Facies 3b, which occurs in close association with Facies F4b. Facies F4c consists of breccia with poorly organized clasts, and most of the clasts appear to be indurated diatom ooze. Internally, individual clasts contain evidence of soft-sediment deformation. Facies F4d consists of dark gray (N 4) interbedded mud and diamict (Table T2). Diamict beds often have gradational lower and sharp upper boundaries. Clasts within the diamict are up to 3 cm in diameter and are predominantly composed of mud clasts. Facies F5a is characterized by dark gray (N 4) diatom ooze (Table T2; Fig. F11G–F11H). By definition, lithostratigraphic units with this facies contain >50% diatoms (see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014]). The remaining constituents in this facies are typically mud, and in some cases calcareous mud is documented. Contacts vary between gradational and sharp. Bed thickness for Facies F5 varies from 20 to 150 cm. Bioturbation is common in this facies, including Zoophycos burrows (Fig. F11G–F11H). Facies F6 is defined by gray (5Y 4/1) to brown (5YR 5/3) volcanic ash (Fig. F13). Bed contacts range from sharp to gradational. Compositionally, this facies consists of 80% glass shards (vitric fragments). The remaining framework grains are feldspar, quartz, and opaque minerals. Dark gray (N 4) volcaniclastic mud and sand define Facies 7 (Fig. F13E–F13F). Bed thickness ranges from 1 to 5 cm. These beds consist of a mixture of volcanic glass (typically 10%–20%), quartz, feldspar, and often diatom microfossils. Beds are often bioturbated. Facies 8 consists of dark gray (N 4) lithified siltstone with abundant calcite cement. These beds occur sporadically and are often associated with poor core recovery. Lithostratigraphic unitsBased on facies associations, five major lithostratigraphic units (I–V) are defined (Table T3; Fig. F10). Units I and V are further divided into subunits. The contacts between lithostratigraphic units at Site U1417 are usually gradational, and the criteria used to define units are discussed below. Unit ISubunit IA
Subunit IA contains dark gray (N 4) mud (Fig. F11A) with subordinate interbeds of volcanic ash as thick as 4 cm (Table T3; Fig. F13A–F13C). Dispersed granule- to pebble-sized lonestones (outsized clasts) are common. Lonestone clasts consist mainly of argillite and metasiltstone, with secondary amounts of metabasalt and minor amounts of granite and sandstone (Fig. F12A–F12D). Greenish gray (10Y 5/1) intervals of diatom-bearing mud with approximately five diatom ooze layers between 20 and 40 cm thick alternate with barren gray (N 5) mud. Volcanic ash consists of >80% vitric (glass) framework grains (Fig. F13). Although volcanic ash (>90% primary volcanic grains; see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014]) comprises <1% of the total described lithology of this subunit, it is considered important. The relative volcanic grain abundance is plotted on the summary diagrams for each hole for reference (Fig. F10). Subunit IB
This subunit shares all lithologic characteristics with Subunit IA; however, it also includes intervals of diatom ooze that occur frequently, with as many as five intervals per core (Table T3; Fig. F11G–F11H). Ooze intervals are generally <10 cm thick. Dispersed granule- to pebble-sized lonestones are more common than in Subunit IA. Silt and sand beds are 2–4 cm thick (Fig. F11B). Volcanic ash (>90% primary volcanic grains; see “Lithostratigraphy” in the “Methods” chapter [Jaeger et al., 2014]) represents <1% of the total described lithology of this subunit and consists of >80% vitric (glass) framework grains. Unit II
This unit is identified by distinct 1–5 cm thick interbeds of fine sand and coarse silt in dark gray (N 4) to greenish gray (10Y 5/1) mud. Mud beds are massive to normally graded and commonly have sharp lower contacts (Table T3). Sand/silt beds occur in the mud at irregularly spaced intervals ranging from 5 to 15 cm. Relative to Unit I, this unit contains fewer lonestones; however, they occur throughout the unit and range up to as many as eight clasts per section. Volcanic ash beds are present, with eight beds distributed over Unit II. Diatom-rich intervals, including thin beds of ooze, similar to those in Unit I occur near the top of Unit II. Unit III
This unit is bounded by the first and last occurrences of 4–8 cm thick beds of muddy diamict interbedded with intervals of bioturbated dark gray (N 4) mud up to 40 cm thick (Table T3; Fig. F11C). Core disturbance makes this relationship difficult to observe in many cases. Diamict intervals contain gravel-sized subangular to subrounded clasts in a muddy matrix and have gradational lower contacts and typically sharp upper contacts. Clast abundance increases uphole in individual beds. Common clast types include argillite and metasiltstone, with secondary amounts of metabasalt. Unit IV
Unit IV contains dark gray (N 4) to dark greenish gray (5GY 4/1) clay-rich mud that is commonly highly bioturbated. Zoophycos burrows and diatom-bearing intervals as thick as 140 cm are common (Fig. F11G). Lonestones are absent. Unit V
Unit V contains predominantly dark gray (N 4) mud with subordinate muddy and sandy diamict, interbedded silt/mud and sand/mud, and diatom ooze (Table T3). Because Unit V is highly diverse, it is divided into 10 subunits to differentiate biosiliceous-rich (diatom ooze) intervals from intervals dominated by terrigenous sediment flux (Fig. F10E). Subunits VA, VC, VE, VG, and VI consist mainly of dark gray (N 4) mud, sandy diamict with lithic and mud clasts up to 3 cm diameter, and plant fragments (Fig. F11D); massive and graded sand beds (Fig. F11E–F11F); interbedded silt and mud; and occasional diatom-rich intervals and diatom ooze. The top of Subunit VA is defined by the first occurrence of a 50 cm thick very dark gray (5Y 3/1) sand bed underlain by a sandy diamict bed. Within the subunit, three other sand beds and six other diamict intervals are interbedded with biosiliceous-bearing mud or diatom ooze. The diamict beds contain lithic and mud clasts, as well as plant fragments. Subunit VC has similar characteristics, with mud clasts and coal in the diamict beds. Subunit VE is defined by diamict containing coal clasts interbedded with dark greenish gray (10Y 4/1) interbedded silt and mud. Subunit VG is defined by the occurrence of normally graded sandy mud beds ranging in thickness from 10 to 60 cm interbedded with mud. These sand beds include mud clasts and granules of coal. Subunit VI is interbedded similarly to Subunit VG, except that the intervals of muddy silt are as thick as 180 cm. Fine-grained Subunits VB, VD, VF, VH, and VJ consist mainly of dark gray (N 4) mud with diatom-rich intervals and diatom ooze, the latter occasionally exceeding 150 cm in thickness. Color banding also can be a characteristic feature of these subunits (Fig. F11H). Of these, Subunit VF is most notable because it includes a breccia bed that is 80 cm thick. In Unit V, ash (Fig. F13) and extensional deformation structures (Fig. F12F) are observed occasionally. Plant debris and terrestrial organic detritus (coal) are components of the sandy diamict (Figs. F11E, F12G–F12J). PetrographyClast lithologiesThe main lithologies of the diamict clasts and lonestones contained in Site U1417 sediment are of low-grade metamorphic origin. In decreasing order of abundance, the lithologies are argillite, metasiltstone, and basalt. Sandstone, siltstone, granite, and quartzite are minor lithologies. These lithologies are unevenly distributed between Holes U1417A–U1417E. The average clast ratio for Site U1417, based on the main lithology types metamorphic (M), sedimentary (S), and igneous (I), is M73S15I12. Bulk mineralogyX-ray diffraction (XRD) analyses were performed on 131 powdered bulk samples from Holes U1417A–U1417E to delineate the bulk mineralogy and identify compositional trends with age or depth in the cores. Diffraction patterns are shown in Figure F14, 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 is uniform downhole, although there are some variations in relative peak intensities, which may indicate slight variations in mineral content. Figure F14A shows the scans for 14 representative samples from this site. The primary minerals identified include quartz, plagioclase (feldspar), mica (muscovite/illite and biotite), and the minerals chlorite and/or kaolinite. Quartz and plagioclase are the dominant peaks, with quartz generally the larger. Figure F14B shows the comparative XRD patterns from 4° to 24°2θ, where scans run before and after the samples underwent a glycol 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 show no indication of expandable clay minerals until ~100 m core composite depth below seafloor (CCSF-B) in the cores. From this depth downhole, we observe a shoulder at ~7°2θ in the samples. After the treatment, a small but distinct peak is observed at ~5°2θ, suggesting the presence of expandable clay minerals. Additional minor mineralogical phases documented in the bulk mineralogy of the cores include calcite associated with well-indurated siltstone intervals and pyrite associated with localized structures such as burrows (Fig. F12K). These phases are most common in lithostratigraphic Unit V. 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. Physical properties characteristics of selected lithologiesWe integrated physical properties data measured using the onboard core logging systems with our sediment core and smear slide descriptions in an attempt to establish relationships that could be used to map the distribution of selected sedimentary attributes and facies at a higher resolution. Sand, volcanic ash, diatom ooze, lonestones, and diamict beds exhibit somewhat unique variations in magnetic susceptibility, bulk density, natural gamma radiation (NGR), P-wave velocity, and color reflectance that can be used to highlight and predict downcore distribution of lithologic features. Because dark gray (N 4) mud is the dominant lithology observed at Site U1417, the physical properties data provided a relatively fast method to identify variations in lithology and identify potential intervals of interest for further study. Figure F15 provides an example of this comparative technique where downcore variations in magnetic susceptibility, bulk density, NGR, and color reflectance are compared to sediment lithology seen in core imagery and smear slide photomicrographs across the lithostratigraphic Subunit IB–Unit II transition in Hole U1417C. Units of diatom ooze centered at 146.6, 147.25, and 148 m CSF-A are lighter gray in core imagery and correspond with relatively low magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and NGR values and relatively high reflectance index values. We use a color reflectance index calculated from the absolute value of a*/b*, which appears to capture the contrast in sediment color across the transitions between mud and ash, ooze, and sand/silt beds. A volcanic ash bed and mafic-rich sand centered between 146.25 and 146.5 m CSF-A result in a complex physical properties data response with respect to magnetic susceptibility. The divergence can be attributed to the difference in mineralogy between the two beds, where the mafic-rich sand has elevated magnetic susceptibility. An interval of interbedded sand and mud centered at 149.8 m CSF-A is well defined by higher magnetic susceptibility and higher color reflectance index values, which mirror the magnetic mineral content of the sand and the change in color across the sand beds, respectively. Lithostratigraphy and depositional environmentsMajor characteristics that define the lithostratigraphic units for Site U1417 are summarized in Figure F16. Unit ILithostratigraphic Subunits IA and IB consist mainly of dark gray (N 4) mud with lonestones and recurring intervals of diatom ooze in Subunit IB (Tables T2, T3). Lonestones are interpreted to be ice-rafted debris deposited from icebergs. The presence of lonestones thus seems to be evidence for tidewater glaciation. Our interpretation is that most of the mud in Unit I originated from suspension settling through the water column and from sediment gravity flows reaching the distal portions of the Surveyor Fan (Powell and Molnia, 1989). Sediment reworking and deposition could also be related to temporal variations in bottom current strength and direction. Recurring intervals of diatom ooze in Subunit IB might be related to one or several of the following processes:
The source for some of the sediment represented by Unit I is interpreted to be the onshore St. Elias and Chugach Mountains located along the southern coast of Alaska. The dominance of low-grade metamorphic lithologies in the lonestones suggests that the Chugach Metamorphic Complex (Gasser et al., 2011) may have been a primary ice-rafted debris source during deposition of both Subunits IA and IB. The fairly even distribution of volcanic ash and volcaniclastic-bearing sand indicates that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have periodic influxes of pyroclastic detritus. Unit IIWe infer that sedimentary processes in Unit II were generally similar to depositional processes in Unit I (i.e., that sediment supply mainly occurred from suspension fallout, sediment gravity flows, and icebergs). However, the increased number of thin beds of fine sand and coarse silt suggests more frequent and more regular deposition from sediment gravity flows (e.g., low-density turbidity flows) (Lowe, 1982) on the distal Surveyor Fan. In addition, the relative absence of diatom ooze intervals may reflect reduced productivity in the water column. This reduced productivity might be caused by one or several factors, including (1) increased sea ice coverage or a larger distance to the sea ice edge, (2) limited nutrient supply, (3) secondary diagenetic processes (such as the dissolution of silica), and/or (4) increased dilution of biogenic-rich intervals by terrigenous sediment. Unit IIIUnit III is characterized by muddy diamict interbedded with bioturbated dark gray (N 4) mud. As in Units I and II, our interpretation is that the muddy sediment was deposited from suspension, as well as from distal sediment gravity flows and icebergs. The lonestones in Unit III are the oldest observed in the sedimentary record at Site U1417 (early Pleistocene or late Pliocene), and they reflect the first appearance at this site of iceberg-derived sediment from calving of tidewater glaciers into the Gulf of Alaska. The accumulation of clasts that form diamict intervals in Unit III in Holes U1417B, U1417D, and U1417E potentially record periods of significantly enhanced ice rafting and/or reduced deposition of mud. Whereas the gradational lower boundaries of the diamict beds most probably mirror a gradual increase in ice rafting, an abrupt termination in ice rafting may have led to the formation of a sharp upper boundary and the onset of “nonglacial” conditions as indicated by the deposition of mud and enhanced bioturbation. The repeated occurrence of couplets of muddy diamict and mud may reflect (1) the recovery of deposits from multiple glacial and interglacial cycles, (2) temporarily increased ice rafting due to enhanced calving from the glacier margin, or (3) dumping from icebergs. Because the pattern of gradual onset and abrupt termination of ice rafting is very similar to “typical” glacial–interglacial cycles, we suggest that the deposits may archive large-scale climatic changes. Further refinement of the shipboard age model may be necessary to confirm this. Unit IVThe main characteristics of Unit IV are intense bioturbation and diatom-bearing intervals, the absence of lonestones, and the presence of isolated fine sand beds. The more intense bioturbation is suggestive of limited terrigeneous sediment influx and/or more productive water column conditions. Unit VUnit V is divided into subunits to distinguish periods of reduced terrigenous sediment supply and higher biological productivity and/or better preservation (diatom ooze; Subunits VB, VD, VF, VH, and VJ) from periods of higher flux of terrigenous sediment (mud; Subunits VA, VC, VE, VG, and VI). Both periods are interrupted by muddy and sandy diamict deposition. The diamict is interpreted to be deposited from sedimentary gravity flows (e.g., high-density turbidity flow) (Lowe, 1982). A distinctive characteristic of Unit V is the presence of coal clasts and woody plant detritus in the sandy diamict facies (Figs. F11E, F12I–F12J). These components are tentatively interpreted to be derived from the coal-bearing Eocene Kulthieth Formation exposed in the onshore thrust belt in the St. Elias and Chugach Mountains (e.g., Plafker, 1987). Other unique characteristics of Unit V are small normal faults (commonly <2 cm of displacement) in the lower parts of Hole U1417E (Fig. F12F). These faults may be related to deformation along the outer rise of the Pacific plate due to subduction at the Aleutian Trench west of Site U1417. Many active normal faults have been mapped in this portion of the Pacific plate and are related to plate-bending processes (Reece et al., 2013). The provenance of the sediment documented in Unit V is unclear. A southeastward restoration of the position of the Pacific plate (i.e., accounting for current northwest plate vectors) may show that these submarine fan systems were supplied with sediment from the coastal mountains of the Yukon Territory and northwestern British Columbia (e.g., the Coast Plutonic Complex). However, more detailed provenance studies are required to verify the locations of onshore sources of sediment for Unit V. |
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