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doi:10.2204/iodp.proc.341.105.2014 LithostratigraphyLithologic summaries of the five holes drilled at Site U1419 are shown in Figure F6. The sediment recovered at Site U1419 contains 14 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 core recovered. Photographs of some of the most common facies are shown in Figure F7. Based on characteristic facies associations, two lithostratigraphic units were defined (Table T3). Facies descriptionFourteen lithofacies were identified and are outlined in Table T2. Most of these are included within the collection of facies observed at other Expedition 341 sites. The numbering of the facies is based on facies documented for all Expedition 341 sites, but only facies documented at Site U1419 are described and discussed here. They include massive mud with lonestones (F1a), massive mud without lonestones (F1b), silt (F2a), interbedded or interlaminated silt and mud (F2b), very fine to coarse sand (F3a), interbedded sand and mud (F3c), interbedded mud and diamict (F4d), clast-poor diamict (F4e), clast-rich diamict (F4f), diatom ooze (F5a), biosiliceous ooze and biosiliceous rich/bearing mud (F5b), calcareous/carbonate-bearing mud (F5c), volcanic ash (F6), and volcaniclastic mud and sand (F7). The greater range in clast abundance within diamicts at this site allowed for differentiation of two subclasses of diamicts (clast-rich and clast poor; see Fig. F6 in the “Methods” chapter [Jaeger et al., 2014a]). These facies reflect deposition from suspension fall out, sediment gravity flows, ice rafting, variations in marine productivity, and volcanic eruptions. The massive and bioturbated mud with lonestones of Facies F1a is mostly dark gray (N 4) to dark greenish gray (10Y 4/1), and bed thickness ranges from 3 to 1082 cm (Table T2; Fig. F7B). Bioturbation is generally absent or, in some cases, slight to moderate. A diagnostic characteristic of Facies F1a is the absence or low abundance of microfossils. Based on smear slides, the composition of the mud is, on average, 70%–80% clay-size particles and 20%–30% silt particles. Minor amounts (<2%) of volcanic glass were documented in this facies based on smear slide observations. Lonestones consist mainly of argillite, siltstone, and metasiltstone with subordinate amounts of granitoid and sandstone and minor amounts of basalt (greenstone) and gabbro (Figs. F7B, F8A–F8F). Facies F1b is identical to Facies F1a, except for the absence of lonestones (Table T2; Fig. F7A), and ranges in thickness up to 432 cm. In both facies, occasional foraminifers were recorded on smear slides. Facies F2a consists of thin beds of dark gray (N 4) to very dark gray (N 3) silt that are irregularly spaced within the mud of Facies F1a and F1b (Table T2). The silt laminae/beds have sharp lower contacts and gradational upper contacts. Individual lamina/bed thickness ranges from 0.2 to 5 cm and often exhibits normal grading; packages of this facies are as thick as 257 cm. Facies 2a is well sorted. The framework grain composition of this facies is almost entirely quartz and feldspar, with few lithic grains identified in smear slides. Accessory framework grains include biotite, hornblende, and heavy minerals. Lonestones are uncommon in this facies but may occur with as many as 30 lonestones per meter. Lonestone lithologies are similar to those described for Facies F1a. Facies F2b consists of very dark to dark gray (N 3 and N 4) to dark greenish gray (10Y 4/1) interbedded/interlaminated silt and mud (Table T2; Fig. F7C). Bed thickness for Facies F2b ranges from 8 to 392 cm. Most commonly the beds contain <40 silt laminae per meter described, but in Section 341-U1419A-10H-3, as many as 80 laminae per meter are documented. Lower contacts of silt laminae/beds are mostly sharp, whereas upper contacts are sharp or gradational. Bioturbation is absent to moderate, and lonestones are present in low abundance. Facies F3a consists predominantly of gray (5Y 5/1) to dark gray (N 4) and dark greenish gray (10Y 4/1) very fine to coarse sand (Table T2; Fig. F7D). This facies often has a sharp lower contact and a gradational upper contact. Bed thickness commonly ranges from 1 to 8 cm but was observed up to 67 cm. The sand has a muddy matrix, is poorly sorted, and sometimes contains minor amounts (<10%) of foraminifers. A few of the sand beds contain 10%–30% foraminifers and are described as foraminifer-bearing sand. The sand framework grains are mainly quartz and feldspar, similar to the coarse silt composition of Facies F2a. Accessory minerals include biotite, hornblende, and zircon. Heavy minerals are generally present but are a minor constituent of Facies F3a. Interbedded sand and mud define Facies F3c (Table T2). This facies is between 4 and 193 cm thick. Graded sand beds are as thick as 3.5 cm and have sharp lower boundaries and sharp upper boundaries. Their spacing is generally <5 cm. Lonestones (<1 cm) are occasionally present. Bioturbation is mostly absent in this facies. Facies F4d consists of dark gray (N 4) interbedded mud and muddy diamict with bed thickness between 53 and 348 cm (Table T2). Individual diamict laminae/bed thickness varies from submillimeter scale to 12 cm. Thicker diamict beds often have gradational lower and sharp upper boundaries. Clasts within the diamict are up to 5 cm in diameter and include siltstone, granitoid, quartz, greenstone, sandstone, and metasiltstone. Diatoms are occasionally present, and visual evidence for bioturbation is absent. Facies F4e is a dark gray (N 4) to very dark gray (N 3, 5Y 3/1) clast-poor diamict with a muddy matrix (Fig. F7E). Bed thickness ranges from 20 to 300 cm. Common clast sizes are granule and pebble, with clasts being subangular to subrounded. Dominant clast lithologies are argillite, siltstone, metasiltstone, granitoid, sandstone, basalt (greenstone), gabbro, chert, and rhyolite (Fig. F8A–F8F). Based on smear slides, the composition of the sand fraction is primarily quartzofeldspathic and the mud fraction is typically 35% silt and 65% clay-size particles. Facies F4f is a dark gray (N 4) to very dark gray (N 3) clast-rich diamict, typically with a muddy matrix (Table T2; Fig. F7F). However, examples of sandy matrix were also documented. Intervals of this facies range in thickness from 10 to 716 cm. Common clast sizes and their lithologies are similar to those described for Facies F4e. Facies F5a is composed of mostly dark greenish gray (10Y 4/1), greenish gray (10Y 5/1), or olive-gray (5Y 4/2) diatom ooze (Table T2; Fig. F7G). At Site U1419, intervals of ooze contain, on average, 80% diatoms, 10% silt, and 10% clay based on smear slides, as well as other minor fossil constituents. Contacts vary between gradational and sharp. The distinction between Facies F5a and F1a or F1b can be very subtle and only recognizable by the documentation of diatom abundance in smear slides. Some examples of Facies F5a also contain minor amounts of foraminifers. Intervals of Facies F5a vary in thickness from 3 to 600 cm. Bioturbation is mostly absent but occasionally slight to heavy, and black mottling is common. Lonestones are rare in Facies F5a, except for in Core 341-U1419C-12H. The biosiliceous ooze and biosiliceous-rich/bearing sediments of Facies F5b are similar to the characteristics described for Facies F5a, except that the biogenic material consists of a combination of diatoms, sponge spicules, and radiolarians and/or the amounts of diatoms are lower than required to be classified as diatom ooze (Table T2). Facies thickness ranges from 11 to 318 cm. Occasionally a few foraminifers were also documented in this facies. Facies F5c contains mostly dark greenish gray (10Y 4/1) calcareous/carbonate-bearing mud and sand (Table T2). Thickness ranges from 3 to 150 cm. Facies F6 is defined by gray (5Y 4/1) volcanic ash (Table T2; Fig. F7H). Bed thickness ranges from 2 to 4 cm. Bed contacts are sharp to gradational. Compositionally, this facies consists of 90% glass shards (vitric fragments). The remaining framework grains are feldspar, quartz, and opaque minerals. Predominantly dark gray (N 4) and dark greenish gray (10Y 4/1) volcaniclastic mud, silt, diamict, and diatom microfossils define Facies F7 (Table T2). Bed thickness commonly ranges from 2 to 529 cm. These beds consist of a mixture of volcanic glass (typically 10%–20%), silt-sized quartz and feldspar, and often diatom microfossils. One feature of the volcaniclastic beds at Site U1419 was the consistent presence of both brown and clear vitric shards. In contrast, volcaniclastic beds at Sites U1417 and U1418 were dominated by clear vitric shards. Lonestones occur but are not common in Facies F7. Bioturbation is mostly absent. Lithostratigraphic unitsBased on facies associations, two lithostratigraphic units were defined (Table T3). The contacts between lithostratigraphic units at Site U1419 are usually gradational, and the criteria used to define units are discussed below. Unit I
Olive-gray (5Y 4/2) to dark greenish gray (10Y 4/1) diatom ooze extends from 0 to 5 m core composite depth below seafloor (CCSF-B) at this site (Fig. F9). Other thinner intervals of ooze (20–50 cm thick) and diatom-rich mud are spaced throughout Unit I. Dark gray (N 4) to dark greenish gray (10Y 4/1) mud with lonestones is the major lithology occurring deeper than 5 m CCSF-B. An interval of clast-rich diamict from 68 to 74 m CCSF-B in Holes U1419B and U1419E corresponds on the CCSF-B scale to mud with abundant clasts in Holes U1419A, U1419C, and U1419D. In Hole U1419A, clast content increases at depths deeper than ~90 m CCSF-B, forming intervals of mud with abundant clasts. This depth is the transition between Units I and II, and it correlates with the transition to diamict described in Holes U1419B–U1419E (Fig. F9). Hole-to-hole variability in lithologic description in terms of the relative abundance of clasts/lonestones is explained by biases associated with the appearance of the core surface after splitting with a wire versus a saw blade. The wire tended to drag the mud across the split-core surface, obscuring the clasts and resulting in an emphasis on mud grain sizes in the visual core descriptions. Cutting with a saw allowed the identification of sedimentary structures, clast distribution, and clast composition at much greater detail. However, intervals of low-density, diatom-rich mud and ooze often were disturbed by the water introduced during sawing. Clast composition is diverse and includes argillite, siltstone, sandstone, and granitoid. Subordinate lithologies include dark gray (N 4) interbedded sand and mud, thin (≤1 cm) sand beds with sharp lower contacts, and interbedded silt and mud. Shell fragments and an articulated bivalve are present in Unit I (Fig. F8G). One bed of volcanic ash was identified across Holes U1419B–U1419D at ~41 m CSF-A (Figs. F6B, F6C, F6D, F9). Another volcanic ash bed was identified in Hole U1419E at ~24 m CSF-A (Figs. F6E, F9). In addition, numerous intervals of volcaniclastic-bearing sediment are present within this unit. Unit II
Dark gray (N 4) muddy clast-poor diamict is interbedded with dark gray (N 4) laminated mud and thin coarse sand beds. 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. Two intervals of diatom ooze occur between 100 and 104 m CCSF-B. One biosiliceous ooze interval occurs at ~170 m CCSF-B. Detailed description of Unit II is limited by incomplete recovery deeper than 114 m CCSF-B, and this is reflected in the site summary diagram (Fig. F9). PetrographyClast lithologiesThe main lithologies of the diamict clasts and lonestones contained in Site U1419 sediment (Fig. F8A–F8F) are, in order of decreasing abundance, siltstone, argillite, sandstone, basalt, granitoid, and chert. The granitoid group includes intermediate and felsic rocks. Metasiltstone, graywacke, rhyolite, quartzite, and gneiss represent minor lithologies. These lithologies are relatively evenly distributed in Holes U1419A–U1419E. Averages for each hole and the site, according to the main lithology types metamorphic (M), igneous (I), and sedimentary (S) (Fig. F10), reveal the slight predominance of sedimentary lithologies over metamorphic and igneous ones. The average clast ratio for Site U1419 is M25I30S45. Bulk mineralogyX-ray diffraction analyses were performed on 17 powdered bulk samples from Hole U1419A to delineate the bulk mineralogy and identify compositional trends with age and/or depth in the cores. The resulting diffraction patterns are shown in Figure F11, and the relative mineral diffraction peak intensities, as defined in “Lithostratigraphy” in the “Methods” chapter (Jaeger et al., 2014a), are listed in Table T4. In general, the bulk mineralogy is uniform downhole, although there are some variations in relative peak intensities, which may indicate slight variations in mineral content. Figure F11A shows the scans for all samples at this site. 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 larger. Figure F11B shows the comparative X-ray diffraction patterns from 4° to 24°2θ, where scans were run before and after the samples had undergone glycolization treatment (see “Lithostratigraphy” in the “Methods” chapter for details [Jaeger et al., 2014a]). This treatment was used to determine the presence of expandable clay minerals (e.g., smectite). The scans suggest evidence for the presence of expandable clay minerals deeper than ~50 m CCSF-B. Our preliminary findings are similar to the results of Molnia and Hein (1982) from modern samples collected on the continental shelf of the Gulf of Alaska. Lithostratigraphy and depositional interpretationsThe distribution of primary sedimentary lithologies and clast abundance at Site U1419 is summarized in Figure F9. Lithostratigraphic Unit I (0–90 m CCSF-B) is primarily characterized by alternation between mud with varying concentration of clasts (dispersed, common, and abundant) and biosiliceous mud and diatom-rich mud. The upper 80 m of this unit also exhibit intermittent layers of sand. A ~5 m thick diatom ooze is found at the very top of the recovered sedimentary sequence, and lonestones are notably absent from this interval. Isolated beds of diatom-rich mud (12–50 cm thick) and a single diatom ooze are found in the upper 70 m CCSF-B of Unit I (Fig. F9). Where counted, the number of clasts >2 mm per described meter fluctuates repeatedly but largely remains below 30 in Unit I. A prominent interval of interbedded mud and silt is found between 54 and 70 m CCSF-B, which is underlain by a 10 m thick interval containing diamict and mud with common and abundant clasts. In addition, multiple diatom oozes and intervals of diatom-rich mud are observed between 70 and 90 m CCSF-B. Counting of clasts was terminated in cores deeper than 70 m CCSF-B because of high concentrations and replaced with clast-rich or clast-poor diamict (Fig. F9). However, the biogenic interval between 80 and 90 m CCSF-B contained sporadic clasts. The transition from Unit I to Unit II (at ~90 m CCSF-B) is defined by the relatively rapid shift from biogenic sediments to the alternation of clast-poor and clast-rich diamict with intervals of mud with common and abundant clasts (Fig. F9). Two intervals of diatom ooze have been identified between 100 and 105 m CCSF-B, but the Unit II sedimentary sequence is largely dominated by poorly sorted mud (with and without lonestones) and diamict. Core recovery averages 18% deeper than 118 m CCSF-B, and recovered sediments largely consist of clast-rich and clast-poor diamict. However, a ~25 cm thick diatom ooze was observed at ~170 m CCSF-B. Unit ILithostratigraphic Unit I consists mainly of dark gray (N 4) mud with lonestones (Fig. F7B). We interpret most of the mud as having originated from suspension settling from turbid meltwater plumes. The lonestones in Unit I are interpreted to have been rafted by icebergs calved from tidewater glaciers (Davies et al., 2011; Powell and Molnia, 1989). Diamict was deposited when icebergs contributed large quantities of debris greater than sand size to this site and/or when the flux of mud was reduced relative to ice-rafted debris, thus increasing the proportion of coarse sediment (Davies et al., 2011) (Fig. F7F). Sediment gravity flows are inferred from the presence of sand beds and intervals with interbedded sand and mud. These sand beds typically have sharp lower contacts and are normally graded (Fig. F7D). Other thin sand beds have less definite contacts and may be deposited from turbid meltwater plumes and/or sea ice. The diatom-rich intervals in Unit I might be related to one or several processes:
The alternation of diatom ooze and diatom-rich sediments with intervals dominated by iceberg or sea ice–transported sediment (mud with lonestones and diamict) is interpreted as reflecting climatic changes (Davies et al., 2011). The sources for lonestones documented in Unit I are interpreted to be the onshore St. Elias Mountains and Chugach Mountains located along the southern coast of Alaska. Metasedimentary lithologies, common in the lonestones, occur in all of these ranges (Plafker, 1987; Plafker et al., 1994; Gasser et al., 2011). Felsic igneous clasts may be derived from the Sanak-Baranoff plutons found dispersed along the southern Alaska margin (Sisson et al., 2003). The thick mud deposits documented in the cores may also reflect supply from the nearby St. Elias orogen that includes fold-and-thrust belts onshore and on the adjacent shelf north of Site U1419 (Worthington et al., 2008; Pavlis et al., 2012). The quartzofeldspathic sand beds are interpreted as potentially being derived by recycling of metasedimentary and sedimentary strata in the accretionary prism complex of the Chugach Range and the Neogene thrust belt of the Yakutat terrane (Plafker et al., 1994). Similar sandstone in the onshore part of the Yakataga Formation of the Yakutat terrane contains detrital zircon populations indicative of reworking from metasedimentary and sedimentary strata exposed in nearby thrust sheets (Perry et al. 2009; Witmer, 2009). The rare volcanic ash and volcaniclastic-bearing sand at Site U1419 indicates that the location was proximal enough to either the Aleutian or Wrangell volcanic belts to have periodic influxes of pyroclastic detritus. The mixing of volcanic ash with other sediment types, common at Site U1419, likely resulted from the high sediment flux from other processes relative to the influx of ash at this site. The common occurrence of brown vitric shards at this site compared to Sites U1417 and U1418 may be a function of the proximity of Site U1419 to the Wrangell volcanic field rather than the Aleutian volcanic belt. Unit IIDiamict Facies F4e and F4f within Unit II are interpreted as having a glacigenic origin. The observation of gradational contacts between facies of clast-rich diamict interstratified with mud (including mud with diatoms) likely indicates a fluctuating sediment supply typical of a glacimarine environment. Massive diamict beds in this unit appear to indicate periods of more intense iceberg rafting with a high flux of sand and coarser sediment. Intervals of interbedded mud and thin diamict beds suggest the possibility of sea ice rafting over Site U1419 in addition to iceberg rafting (Cowan et al., 1999). Sediment gravity flows are indicated by interbedded sand and mud and thicker sand beds. Some of the diamict intervals may have been deposited by debris flows, but this potential transport mechanism requires postcruise study. Two intervals of diatom/biosiliceous ooze are interbedded with diamict in Unit II, suggesting that episodes of higher productivity and/or reduced glacial sediment supply also occurred. |
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