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doi:10.2204/iodp.proc.302.104.2006 LithostratigraphySediments recovered from Expedition 302 Sites M0002, M0003, and M0004 span 0 to 427.6 mbsf and range in age from Holocene to Late Cretaceous. There is no lithologic description for Site M0001, as this site was abandoned after recovering a single core catcher. The suite of sites are treated as a single stratigraphic section based on correlative seismic signatures, logging and physical properties, chemostratigraphies, lithostratigraphies, and biostratigraphies of these closely spaced sites. Four lithologic units (Table T3) are defined on the basis of color, texture, and compositional variations identified from visual core description and smear slide analysis (Figs. F1, F2), as well as X-ray diffraction (XRD) (Fig. F3) and total organic carbon (TOC) content analyses (Fig. F40). A first-order observation of downcore compositional variations, which helped define the lithostratigraphic units, is a shift from siliciclastic-dominated (Unit 1) to biogenic-dominated (Unit 2) and back to siliciclastic-dominated (Unit 3) lithologies. Unit 4 was distinguished from Unit 3 based on an abrupt coarsening of the sediment texture following an interval of poor core recovery. Second-order observations of a textural and compositional nature, including variation in the amount of sand, biogenic carbonate and silica, pyrite, TOC, and a downcore shift in sediment color from generally brown to olive, back to brown, and then to gray and black, enabled further differentiation of subunits within Unit 1. Lithostratigraphic Unit 1
Unit 1 (0.01–220.24 mbsf; Hole M0002A) is characterized by siliciclastic sediments, color banding, sandy lenses, and isolated pebbles. The dominant lithology of Unit 1 is silty clay. The top of Unit 1 contains small amounts of biogenic carbonate, whereas the base of Unit 1 contains small amounts of biogenic silica (Table T3). Smear slide analyses indicate that clay minerals, quartz, and feldspar constitute the major siliciclastic components. Other mineral components include pyrite, accessory minerals, mica, opaque minerals, and siderite. Volcanic glass is also present in trace amounts. TOC content generally increases downcore in Unit 1, ranging from <0.5 wt% near the top of Unit 1 to 2–3 wt% at the base (see “Geochemistry”). Isolated pebbles occur throughout Unit 1 (Table T4). Unit 1 ranges in color primarily from brown to olive and back to brown above 192.94 mbsf (Section 302-M0002A-44X-1, 95 cm) and is gray to very dark gray or black below this level. Color banding of variable thicknesses (centimeters to meters) is common throughout the upper 196.2 mbsf (Section 302-M0002A-45X-1, 24 cm). These color bands generally have horizontal sharp to gradational (mottled) contacts. From 196.2 to 198.13 mbsf (Section 302-M0002A-46X-1, 113 cm), color banding is quite different from the overlying sediments. This interval is characterized by variably tilted, black to gray couplets, often with sharp basal contacts, which truncate underlying couplets. From 198.13 mbsf to the base of Unit 1, color banding is absent. The abrupt downcore color shift to darker (grayer) lithologies at 192.94 mbsf is associated with a gradual increase in microconcretions and pyrite content and decrease in millimeter- to centimeter-scale sandy lenses (absent below Section 302-M0002A-46X-1, 58.5 cm; 197.59 mbsf). Dark gray to black millimeter- to gravel-size concretions first occur at 131.09 mbsf (Section 302-M0002A-29X-2, 141 cm). Distinct millimeter- and centimeter-size metallic yellow pyrite concretions and burrow infillings then appear from Section 302-M0002A-44X-2, 27 cm, through 44X-4, 12 cm (195.93 mbsf). Concretions are absent in Unit 1 below this depth. However, disseminated pyrite is a significant bulk mineral component of Unit 1 downcore from 198.13 mbsf (Section 302-M0002A-46X-1, 113 cm) according to smear slide and XRD data (Figs. F2A, F3). Unit 1 is divided into six subunits reflecting these changes in texture, composition, and color. Subunit 1/1
Subunit 1/1 comprises silty clay, silty mud, and sandy mud with strong color banding including brown (10YR 4/3), yellowish brown (10YR 5/4), light olive-brown (2.5Y 5/4), light reddish brown (5YR 6/4), gray (2.5Y 6N 6/0), pale brown (10YR 6/3), light yellowish brown (2.5Y 6/4), dark brown (10YR 4/3), olive-gray (5Y 4/7), olive (5Y 5/4), dark yellowish brown (10YR 4/6), and dark gray (10YR 4/1). Color bands range in thickness from 2 to 50 cm, generally with sharp contacts between them. Bioturbation is slight throughout the unit. Smear slide analyses indicate minor amounts of biogenic carbonate. Isolated pebbles (0.5–1.0 cm in diameter) occur throughout the subunit. TOC content is <0.5 wt%. Subunit 1/2
The main lithologic distinction between Subunits 1/1 and 1/2 is textural. The major lithology of Subunit 1/2 is silty clay and therefore contains less sand than Subunit 1/1, which includes lithologies of silty clay, silty mud, and sandy mud. Light olive-brown (2.5Y 5/4) silty clay with olive-brown (2.5Y 4/4) thin to medium banding is present in the upper part of Subunit 1/2, and dark brown (10YR 4/3) and yellowish brown (10YR 5/4) silty clay with centimeter-scale, very dark gray mottling is present in the lower part of Subunit 1/2. Isolated pebbles and millimeter- to centimeter-scale sand lenses are present throughout. A 7 cm interval of yellow (10YR 7/6) clay with a sharp upper and gradational basal contact is present in interval 302-M0003A-1H-3, 107–114 cm. Light olive-brown (2.5Y 5/4) silty mud is present in minor amounts in interval 302-M0004C-4H-1, 74–68 cm. This subunit is slightly to moderately bioturbated throughout. A Thalassinoides burrow is present in interval 302-M0003A-1H-3, 10–20 cm, and several Chondrites burrows are present in Core 302-M0004C-3H. Minor amounts of biogenic carbonate are present in Subunit 1/2. TOC content of Subunit 1/2 is <0.5 wt%. Drilling disturbance is common, with slurry and flow-in in Sections 302-M0002A-3X-1 through 3X-4 and 4X1-1 through 4X-4. Subunit 1/3
The lithostratigraphic distinction between Subunit 1/3 and overlying Subunit 1/2 is primarily based on downcore sediment coarsening, a brown to olive color transition, and an absence of biogenic carbonate. Silty clay and silty mud with alternating olive (5Y 5/3, 5Y 4/3, and 5Y 5/6), gray (5Y 5/1), olive-gray (5Y 5/2 and 5Y 4/2), olive-brown (2.5Y 4/4), dark gray (2.5Y N4/1, 2.5Y N4/0, and 5Y 4/1) and light olive-brown (2.5Y 5/4) color bands are found at scales ranging from decimeters to meters, sometimes with mottled contacts between them. Some intervals are characterized by thin centimeter-scale color banding. Short intervals of gray (5Y 5/1) to olive-brown (2.5Y 4/4) sandy mud occur in intervals 302-M0002A-32X-2, 11–18 cm, 32X-3, 26–32 cm, and 302-M0004C-6X-2, 46–47 cm. Millimeter- to centimeter-scale sand lenses and isolated pebbles occur throughout. Millimeter-scale dark gray to black microconcretions are occasionally present from Section 302-M0002A-29X-2, 141 cm, through the base of the subunit. An interval of firm sediment occurs from Section 302-M0002A-30X-1, 0 cm, through 31X-CC. Bioturbation is slight to moderate with well-defined Chondrites burrows in Cores 302-M0002A-17X, 20X, 21X, 32X, and 35X. TOC content is <0.5 wt%. Drilling disturbance in recovered cores is generally minor, although moderate disturbance (biscuiting), slurry, and flow-in occur in discontinuous intervals in Cores 302-M0002A-6X through 12X, within parts of Cores 302-M0002A-16X, 27X, and 32X, and within parts of Cores 302-M0004C-8X through 9X. Subunit 1/4
The lithostratigraphic distinction between Subunit 1/4 and overlying Subunit 1/3 is based on a downcore color change from olive back to brown (Fig. F4; somewhat similar to Subunit 1/2), in addition to a slight downcore sediment coarsening, increase in the presence of gray and black microconcretions, and decrease in TOC content. Subunit 1/4 lithology comprises silty clay, which alternates between dark brown (10YR 4/3 and 10YR 3/3), very dark grayish brown (10YR 3/2), very pale brown (10YR 7/3), and pale yellow (2.5Y 7/4) intervals at scales of decimeters to meters. Millimeter- to centimeter-scale sand lenses (e.g., interval 302-M0002A-38X-4, 100–101 cm) and isolated pebbles are present throughout this subunit. Millimeter-scale dark gray to black microconcretions are common in Core 302-M0002A-39X and below. Bioturbation is slight throughout Subunit 1/4 and includes a Chondrites burrow in Section 302-M0002A-42X-3. The brown sediments of Subunit 1/4 between ~170 and ~193 mbsf are barren of all palynomorphs. TOC content is <0.2 wt%. Subunit 1/5
The lithostratigraphy of Subunit 1/5 is distinguished from overlying Subunit 1/4 in several ways, including a downcore shift in general sediment color from brown to gray and black, a change to an unusual cyclic color banding pattern that resembles “zebra stripes,” a downcore shift from gray/black microconcretions to metallic yellow pyrite microconcretions, and an increase in TOC content. The contact between Subunits 1/4 and 1/5 at 192.94 cm is sharp (Fig. F5). The top of Subunit 1/5 is characterized by light gray (2.5Y N7/0), gray (2.5Y N5/0), and olive-gray (5Y 5/2) silty clay. This interval is slightly bioturbated and contains metallic yellow millimeter- to centimeter-scale pyrite microconcretions. The majority of this subunit is dominated by tilted color bands, or couplets, 0.5 to 3 cm thick, of very dark gray (5Y 3/1) and black (5Y 2.5/1) firm silty clay. The top of this zebra-stripe interval begins at Section 302-M0002A-45X-1, 24 cm, and continues to the base of the subunit. A 0.5 cm thick layer of dark brown (10YR 4/3), hard silty clay (interval 302-M0002A-45X-1, 27.5–28 cm) is present close to the top of the zebra-stripe interval. Packages of couplets within the zebra-stripe interval are tilted, and bases of many of these sediment packages are sharp and truncate (crosscut) the underlying layers as the tilting direction changes (Fig. F6). Within individual tilted packages, upper layers also appear to crosscut underlying layers, but some gradational contacts exist as well. The thickness of the couplets and the angle of tilt increases upsection. Bioturbation is absent in the zebra-stripe interval. Millimeter- to centimeter-scale sand lenses and isolated pebbles occur throughout Subunit 1/5. TOC content of Subunit 1/5 is 0.75–1.0 wt%. Subunit 1/6
The lithostratigraphy of Subunit 1/6 is distinguished from overlying Subunit 1/5 by a downcore change to very dark gray (5Y 3/1), firm to very firm, homogeneous silty clay to clayey silt. Smear slide and XRD data indicate that an abrupt increase in pyrite defines the boundary between Subunits 1/5 and 1/6. XRD data further indicate that a downcore increase in the ratio of kaolinite to chlorite and a decrease in quartz also mark this subunit boundary (Fig. F3). Black millimeter-scale mottling is present in Sections 302-M0002A-46X-3, 36 cm, and 48X-2, 34 cm. A piece of hard very dark grayish brown (10YR 3/2) material, possibly coal, is present in interval 302-M0002A-46X-2, 51–53 cm. Isolated pebbles are present throughout this subunit. Smear slide analyses indicate that siliceous microfossils (diatoms, ebridians, and silicoflagellates) are present in minor amounts toward the base of this Subunit. TOC content of Subunit 1/6 is 2–3 wt%. Moderate drilling disturbance (biscuiting) is common in Core 302-M0002A-48X through Section 50X-1, 29 cm. Slurry occurs below this level to the base of the unit. Lithostratigraphic Unit 2
Unit 2 (220.24–313.61 mbsf) is dominated by very dark gray (5Y 3/1) mud-bearing biosiliceous ooze. Submillimeter-scale light and dark laminations occur throughout Unit 2, occasionally exhibiting cross bedding that is visible in both undisturbed intervals and within coherent biscuits (e.g., intervals 302-M0002A-60X-3, 60–98 cm, and 302-M0004A-6X-4, 34–38 cm). No bioturbation is visible throughout the unit. Isolated pebbles are observed as deep as Section 302-M0002A-55X-4, 122 cm (239.34 mbsf). Smear slide analyses indicate that dominant components include biosiliceous matrix and microfossils (abundant diatoms, common to rare ebridians, and silicoflagellates). Siliciclastic components are minor. Unlike Units 1, 3, and 4, XRD data indicate that Unit 2 is characterized by the dominance of K-feldspar over plagioclase (Fig. F3). Millimeter-scale pyritized lenses occur in Sections 302-M0002A-52X-2, 5 and 10 cm. TOC content in Unit 2 is generally between 2 and 3 wt%. Within recovered sections, drilling disturbance is highly variable, ranging from undisturbed to slightly rotated drilling biscuits to drilling slurry. Lithostratigraphic Unit 3
The primary distinction between Units 2 and 3 is the major lithology of mud-bearing biosiliceous ooze (Unit 2) in contrast to clay (Unit 3). In several ways, Unit 3 resembles Subunit 1/6; both are dark gray fine-grained siliciclastic-dominated transitional lithologies that contain pyrite and minor amounts of biogenic silica toward their contacts with Unit 2. Smear slide analyses indicate that siliceous microfossils are present in rare amounts in Sections 302-M0004A-15X-CC, 18X-CC, and 19X-CC. XRD analysis suggests that biogenic opal may be present down to Section 302-M0004A-26X-CC. Geochemical analyses indicate that the fine-grained sediment in the upper part of Unit 3 (to ~350 mbsf) is largely authigenic (silica altered to crystoballite), with only minor terrigenous input. These distinct geochemical changes in Unit 3 are described further in “Geochemistry.” The top of Unit 3 comprises gray (5Y 3/1) to very dark gray (5Y 4/1) firm to very firm clay. From Core 302-M0004A-31X through Core 34X, the sediment coarsens slightly and is dominated by a gray (5Y 3/1) to dark olive-gray (5Y 3/2) silty clay. A 3 cm interval of yellowish brown (10YR 5/4) sandy mud is present in interval 302-M0004A-32X-CC, 6–9 cm, and Section 33X-1 is a very disturbed, dark olive-gray (5Y 3/2) silty mud. Dark olive-gray (5Y 3/2) clay is again present from Core 302-M0004A-35X to the base of the unit. Smear slide analyses indicate that the dominant components in the terrigenous clay and silty clay are, in increasing order, feldspar, pyrite, quartz, and clay minerals. Accessory minerals include minor amounts of zircon. Submillimeter-scale laminations are present in Cores 302-M0004A-19X through 32X. Millimeter to centimeter-scale pyrite concretions occur in Cores 302-M0004A-27X, 28X, 30X, 32X, 34X, and 35X. A concretion is present near the base of the unit (302-M0004A-31X-CC, 48–50 cm), but this may be a product of core disturbance and could represent part of a harder underlying interval that was not well recovered. Slight bioturbation occurs in parts of Cores 302-M0004A-27X, 28X, 29X, 30X, 34X, and 35X. TOC content in Unit 3 is generally between 1 and 2.5 wt%. Drilling disturbance (biscuiting) is moderate in Cores 302-M0004A-19X to 34X. Lithostratigraphic Unit 4
Loose coarse sand was recovered in Section 302-M0004A-39X-CC and was bagged at sea. Because the stratigraphic position of this sand cannot be constrained, Unit 4 begins at the top of Core 302-M0004A-41X, where a 4 cm sandstone fragment was recovered. Dark olive-gray (5Y 3/2) clayey mud and very dark gray (5Y 3/1) silty clay to clayey mud are the dominant lithologies in Unit 4, occurring from Sections 302-M0004A-41X through 42X-CC, 10 cm. Very dark gray (5Y 3/1) homogeneous silty sand occurs in the deepest 7 cm of the unit. Smear slide analysis indicates that Unit 4 is dominated by quartz, feldspar, and clay minerals. Submillimeter to centimeter-size pyrite concretions are present in Sections 302-M0004A-41X-1 and 42X-1. Centimeter-size gravel is present at the base of 42X-1, but this may be a product of core disturbance. The TOC content of Unit 4 is 1 wt%. The entire unit is highly disturbed by drilling. SummaryFine-grained siliciclastic sediments (clays to silty muds), ranging in age from Holocene to Late Cretaceous (0–428 mbsf), are the dominant lithogenic material recovered in the suite of Expedition 302 sites. This is interrupted in the middle Eocene by dark gray laminated mud-bearing biosiliceous ooze (~220 to ~313 mbsf; Unit 2). This ooze is also characterized by elevated amounts of TOC and algal-derived organic carbon (see “Organic geochemistry”). Dark, organic-rich and biosiliceous laminated units are indicative of high productivity but relatively low oxygen bottom waters. The largely fine grained texture of the siliclastic sediments is indicative of predominantly low energy marine environments throughout most of the Cenozoic. However, the fine-grained zebra-stripe interval within Subunit 1/5 is an exception. Cores within Subunit 1/5 (302-M0002A-44X, 45X, and 46X) give ample evidence of rapid changes in depositional environment with distinct breaks in layering, crosscutting, and cyclic changes in color. Seismic reflection data taken near the sites (Fig. F7B) indicate an interval of high-frequency, anastomosing reflections (see Jakobsson et al., this volume), equivalent to this zebra-stripe interval. Both the character of the bed forms and their apparent seismic signature suggest a dynamic sedimentary environment during part of the middle Miocene(?). This “cut and fill” character may indicate migrating sediment waves (Hall, 1979). Currents that could create such an energetic environment include strong baroclinic or boundary currents (Aagard and Carmack, 1994), tidal-driven turbulence on top of and around the ridge, and the oscillation of internal waves on a surface with a high density gradient. Sand lenses are observed as deep as 198 mbsf (bottom of Subunit 1/5, early Miocene). Isolated pebbles are present throughout Unit 1, with the deepest one observed, visually, in Unit 2 (302-M0002A-55X-4, 122 cm; 239.34 mbsf; middle Eocene). The sand lenses and isolated pebbles are interpreted as ice-rafted debris, suggesting that at least seasonal ice has been present in the central Arctic since the middle Miocene and perhaps as early as the middle Eocene. Lithologic unit changes do not necessarily coincide with hiatuses interpreted on the basis of biostratigraphy. However, a major hiatus is likely within Core 302-M0002A-46X near the Subunit 1/5 and 1/6 boundary. Although color is used as a factor in defining lithologic units, color changes do not always coincide with mineralogical and textural changes, suggesting a strong diagenetic influence on color banding in these sediments. |
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