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doi:10.2204/iodp.proc.329.103.2011 LithostratigraphyThe sediment at Site U1365 is primarily clay, chert, and porcellanite. The principal components of the clay are clay minerals, zeolite, and red-brown to yellow-brown semiopaque oxide (RSO) (see Site U1365 smear slides in “Core descriptions”; Fig. F7). Preliminary X-ray diffraction (XRD) results of the clay indicate that its principal components are minerals from the smectite and mica groups (Fig. F8). The zeolite found at Site U1365 is likely phillipsite, an alteration product of volcanic glass that is abundant in most Pacific Ocean seafloor sediment (Glaccum and Bostrom, 1976). RSO is an iron manganese hydrated substance found in amorphous (Heath and Dymond, 1977) and crystalline (Kastner, 1986) phases. Although its processes of formation and diagenetic alteration are incompletely defined (cf. Glasby, 1991), RSO is commonly associated with very low sedimentation rates in pelagic marine environments (Kastner, 1986; Quilty et al., 1976). The porcellanite and chert at Site U1365, as at neighboring DSDP Site 596, contain abundant radiolarians (Shipboard Science Party, 1987). Based on compositional and textural attributes, the sediment at Site U1365 is divided into three lithologic units (Fig. F9). Unit I is zeolitic metalliferous pelagic clay that is divided into Subunits IA and IB based on the vertical distribution of zeolite. In Subunit IA, zeolite tends to increase with increasing depth. Zeolite concentrations in smear slides increase in Subunit IA from 10% at the surface to 30% at 8 mbsf in Hole U1365A to 9 mbsf in Hole U1365D. Subunit IB is defined by a similar trend in zeolite concentration: ~0%– 5% at the upper boundary and increasing to ~25% at the lower contact with Unit II, the interval of porcellanite and chert. The base of Unit II is defined by the transition from porcellanite and chert-dominated strata to the metalliferous clay of Unit III. This lowermost sedimentary unit is easy to distinguish from the other units by its distinctive black color and its very high RSO content. Description of unitsUnit ISubunit IA
All but the lower 40 cm of Subunit IA is dark brown (7.5YR 3/3) (Fig. F10, F11A). The base of the interval corresponds to the base of an interval of gradational color change. Colors change downcore from dark brown to very dark brown (7.5YR 2.5/2). The lower limit of the color change in Hole U1365A is at interval 329-U1365A-2H-2, 50–90 cm. A similar color change occurs in Section 329-U1365B-2H-4 above and below 48–70 cm. Whole-round sampling prior to observation obscured the depth of contact in this hole. In Hole U1365C, the interval containing the lower contact was not recovered. Hole U1365D was cored to obtain the section that was missed in Hole U1365C. Whole-round samples were removed prior to inspection of the sediment from Hole U1365D, like Hole U1365B. The likely location of the lower contact is in Section 329-U1365D-2H-1 between 50 and 60 cm. Color changes do not correspond to changes in modal composition of minerals based on smear slide analyses. Infrequent pale brown sediment is discussed below. Smear slide analyses identify clay, RSO, and zeolite components (Fig. F10A). From top to bottom, clay concentrations decrease from 30% to 20% and zeolite abundance increases from 10% to 40%. The higher zeolite abundances are associated with diffuse pale brown (10YR 8/4) and brown (7.5YR 5/4) layers that are as thick as 9 cm (see core photographs of intervals 329-U1365A-1H-3, 88–97 cm; 1H-4, 125 cm; and 2H-1, 41–43 cm, in “Core descriptions”). Clay minerals identified by XRD analysis of the <2 µm particle size fraction are in the smectite group. Chlorite was also prevalent among the <2 µm particles. Zeolite crystals observed in smear slides are euhedral and prismatic and have long axes lengths of 10–100 µm (Fig. F10A). RSO grains are rounded to irregular shapes, range from 1 to 80 µm widths, and constitute 15%–25% of the sediment. No clear trends are evident in the RSO distribution. Pebble-sized manganese nodules are present at the top of Subunit IA in all cores (Fig. F11C). Black laminations, disturbed by coring, occur at intervals 329-U1365A-1H-CC, 12 cm, and 2H-1, 99 cm. At nearby Site 596, shipboard scientists identified similar features as manganese-rich hardgrounds (Shipboard Scientific Party, 1987). Consolidation of the sediment in Subunit IA transitions from very poorly consolidated near the mudline to poorly consolidated at the base of the subunit. Moist samples are sticky. The structures in most sections are homogeneous and without bedding features. Mottling increases with depth, although the variations in ichnofabric intensity are all within Class 2 (slight bioturbation). Mottling is most evident where heterogeneous components are mixed (e.g., intervals 329-U1365A-1H-3, 88–97 cm, and 2H-1, 101–106 cm). Subunit IB
Well over 90% of Subunit IB is very dark brown (7.5YR 2.5/2 and 7.5YR 3/2) (Fig. F11B). The color is very uniform and gradations from top to bottom of this interval are not apparent except in spectrophotometric data acquired on the shipboard Section Half Multisensor Logger (SHMSL). The integrated spectral response b* shows both gradients and stepwise subtle changes in the dark brown color of the core that cannot be recognized by simple visual observation (see visual core description of Core 329-U1365A-3H in “Core descriptions”). These changes are, however, very subtle and do not likely indicate any significant change in composition or texture. Two color changes that represent only a small fraction of the sediment are considered important variations from the typical color of Subunit IB. First, very pale brown (10YR 8/4) colors are associated with very thin beds and laminations throughout the lower half of this subunit (Fig. F11D). Where very pale brown and dark brown sediment blend together, the resulting color is brown (7.5YR 5/4; see core photograph of interval 329-U1365A-4H-5, 26 cm, in “Core descriptions”). Second, shades of black (2.5Y 2.5/1) and very dusky red (10R 2.5/2) are interbedded with very pale brown (10YR 8/4) discontinuous laminations, diffuse beds, and rounded to subangular lenses. Similar to Subunit IA, smear slide analyses of Subunit IB indicate the presence of clay, RSO, and zeolite components (Fig. F7). Clays and RSO fluctuate without easily defined trends; each component constitutes 10% to 50% of the total observed particles in any given smear slide. Zeolite minerals markedly increase in concentration with depth. Proportions of zeolite in the upper sections are between 0% and 5%, whereas zeolite proportions near the base are between 20% and 25% and constitute 50% of the particles in smear slides made from pale brown laminations. Four distinct thin beds of porcellanite exist in the lowermost meter of Subunit IB in Hole U1365A (Fig. F11D). Three lithologically similar beds are found in a similar stratigraphic position in Hole U1365B and two are found in Hole U1365C. A single manganese nodule was recovered from Section 329-U1365A-3H-1 at 16 mbsf. Consolidation in Subunit IB is uniformly and moderately indurated (i.e., firm clay) except in the porcellanite intervals. The porcellanite beds are very highly indurated. Small vertical displacements of sediment occur in intervals 329-U1365A-4H-3, 145–150 cm; 329-U1365B-4H-5, 114–124 cm; and 329-U1365A-5H-3, 72 and 88 cm. Inclined laminations and bedding (beyond those expected with APC coring or rotating indurated porcellanite fragments) are in interval 329-U1365A-5H-3, 68–80 cm. Wispy laminations at interval 329-U1365A-5H-3, 89 cm, resemble soft-sediment deformation structures. Radiolarians were found in the metalliferous clay near the base of Subunit IB (Sample 329-U1365A-5H-4, 39 cm, and 5H-CC). Subsequent biostratigraphic analyses place these fossils in the mid-Maastrictian age (see “Paleontology and biostratigraphy”). Unit II
Chert colors in this unit vary from very dark gray (N 3/1) and dark greenish gray (N 4/1) to dark bluish gray (5PB 4/1) (Fig. F12A). The porcellanites are very pale brown (10YR 8/4). Pelagic clays and silicified clays are uniformly dark brown (7.5YR 3/2) (Fig. F12B). Chert and porcellanite were identified by their physical appearance and confirmed by XRD analyses (Fig. F8C). Because most cores failed to recover intact chert intervals, slow drilling and short (i.e., <1 m) core recoveries were used to infer the distribution of these lithologies in Unit II. When interbedded pelagic clay was recovered, smear slides were prepared and analyzed. Clay was found only in the lower half of Unit II in Cores 329-U1365A-14H, 15H, 19H, and 22H. The amount of clay in these samples, on average, is several percent lower than that observed in Unit I. On average, zeolite concentrations are also lower than those observed in the overlying sediment of Subunit IB. The average concentration of RSO through the cherty interval is 10% above the average value of RSO in Unit I (Fig. F7). Radiolarians were found in cherty and metalliferous clay-rich intervals of Unit II. Those fossils found in Section 329-U1365A-14H-CC are mid-Campanian age (see “Paleontology and biostratigraphy”). Some of the larger chert fragments contain intercalated thin to very thin porcellanite beds. Porcellanite fragments, in turn, include very thin beds of highly indurated pelagic clay. Although many chert fragments are massive, several pieces contain relict laminations. Several larger chert fragments exhibit fracturing unrelated to drilling and coring. Occasionally, the fractures show orange alteration zones near the fracture. Laminated pelagic clay is grouped into the equivalent thickness of beds (i.e., >10 cm) in interval 329-U1365A-14H-1, 125–147 cm, and between interval 15H-1, 90 cm, and Section 15H-CC that are bound by chert. In Hole U1365A, contacts for the unit and clay/chert pairings within Unit II are all sharp. Contacts in Holes U1365B and U1365C are less well defined because drilling was employed to avoid the chert. Unit III
Unit III contains several dark colors. The dominant colors are brown-black (7.5YR 2.5/1) and red-black (5YR 2.5/) (Fig. F13A). Individual beds and laminae have various colors that are interpreted as shades of gray, green, brown, very pale brown, and red. Several beds possess a distinct coloration not seen elsewhere in the stratigraphic sections. The beds’ color exists somewhere between black to dark reddish gray (N2.5 to 2.5YR 3/1) and dark greenish gray (10G 3/1) (Fig. F13B). Thin, bright red laminations (10R 4/6) overlie very pale brown clay-rich laminations, lenses, and beds at several locations in the lower one-third of Unit III. Smear slide analyses indicate Unit III is replete with RSO (Fig. F7). Five of seven slides prepared from samples below Section 329-U1365A-24H-1 (68 mbsf) contain >90% RSO. The highest RSO concentrations are in the dark reddish to dark greenish gray bed in intervals 329-U1365A-24H-3, 146 cm, to 24H-4, 14 cm (Fig. F10B). Microscopic inspections of the metalliferous clay in intervals 24H-3, 144 cm, and 24H-CC, 10 cm, revealed ash layers containing 100% volcanic glass. The dark reddish to greenish gray clay beds are found in Holes U1365A–U1365C, although the number of beds varies from two in Hole U1365C to three in Hole U1365A to four in Hole U1365B. All three holes also contain volcanic glass–bearing clay. XRD results confirm the absences of both smectite and chlorite but show no clear indications of either the (assumed amorphous) RSO or volcanic glass. Metalliferous micronodules are present in concentrations of <1%. Chert gravel occupies the Section 1 tops of three cores recovered from Holes U1365A–U1365C. The brown-black and red-black metalliferous clays that make up most of Unit III are very well consolidated. The dark reddish to dark greenish gray clay (with very high RSO content) exhibits a high degree of plasticity. The dark reddish to dark greenish gray clays contain inclined, very fine laminations of light greenish gray (N 8/1) clay. These laminations are discontinuous across the width of the section-half core face. An unusual disturbance, showing compressional displacement similar to a reverse fault, exists along a plane that enters Section 329-U1365A-24H-3 at 143 cm, where it causes the offset of inclined fine lamination by ~5 mm. Moving upward along this plane, beds of dark greenish gray (10G 3/1) clay at 140 and 119 cm are displaced by 9 and 12 cm, respectively. Elsewhere, mottling is present in several short sections of Unit III. The intensity of the bioturbation is index Class 1, <30% disturbance of the sediment interval. The layer of chert gravel at the top of each core in Unit III is interpreted to be fall-in from Unit II. Sediment/Basalt contactComponents of the sediment/basalt interface were recovered from Hole U1365A. The interface spans intervals 329-U1365A-25H-2, 131–137 cm, to 25H-CC, 0–5 cm (75.2–75.32 mbsf). The underlying (and last) core of Hole U1365A (26H) comprised “fall-in” chert and included no basalt. The sediment at the contact was highly disturbed and consisted of a mixture of fragmented basalt and black clay. The composition of the sediment immediately above the basalt is red-black (5YR 2.5/1) metalliferous clay with bright red (10R 4/6) and very pale brown (10YR 8/4) clay lenses (Fig. F13C). These lenses are horizontal but discontinuous across the width of the core. Some of the disturbances in the variegated lenses resemble burrows. Unlike the overlying metalliferous clay, the variegated lenses are friable and form irregular aggregates with 2–4 mm diameters but contain no silt or sand grains. The fragments of basalt found in intimate association with the clay are altered. The basalt is described in “Igneous lithostratigraphy, petrology, alteration, and structural geology.” DiscussionSediment composition and textureSeven XRD analyses were performed on the <2 µm separates of samples intended to represent average lithologies of each unit and the interval of gradation between Subunits IA and IB. The minerals tentatively identified in these analyses include smectite group minerals (nontronite and/or beidellite and montmorillonite) and chlorite. Together, these minerals constitute 20%–40% of the sediment in Unit I, 5%–40% of the sediment in Unit II, and <5% of the sediment in Unit III (Fig. F7). Variations in clay abundance in Unit II are inversely proportional to the abundance of RSO. In chert, the abundance of clay and microfossils is high (i.e., 80%–90%) and the RSO abundance is low (i.e., 10%–20%). In interbedded clay, RSO content is 70%–90% of the mineralogy and clay and microfossil content is only 5%–10%. The overall trend of decreasing clay abundance with increasing depth and the occurrences of smectite and kaolinite are similar to neighboring Site 596 and the surrounding southwestern Pacific region (Graham et al., 1997; Shipboard Scientific Party, 1987). However, the absence of illite in our results is in stark contrast to other results from the southwestern Pacific Ocean (Graham et al., 1997). RSO is present in all but 3 of the 45 smear slides prepared for Hole U1365A. Smear slides show many of the RSO grains fall in the 5–50 mm particle size range. Beds in which RSO is absent contain high proportions of volcanic glass or quartz grains. XRD analyses were unable to detect any unique peaks associated with RSO and thus imply the material is amorphous at this site. RSO content approaches 100% of the smear slide grain populations in clay near the sediment/basement interface. The abundance of RSO is not without precedence: Site 596 reports include smear slide tallies of as much as 90% RSO in basal metalliferous clays at that location (Shipboard Scientific Party, 1987). The abundance of RSO in Unit III implies either prolonged low sediment accumulation rates or postdepositional alterations that promoted RSO abundance by removing less stable minerals and glasses (Heath and Dymond, 1977). The average abundance of zeolite is between 15% and 20% of the mineral content in Unit I and the upper half of Unit II. Zeolite abundance falls to 0%–5% in Unit III. Zeolite crystal particle size spans a large range, from <5 to >100 µm. XRD scans indicate the zeolite crystals in the 2–38 µm size split are most likely phillipsite, although the variety of phillipsite, its crystallinity, and the co-occurrence of other members of the zeolite group were not assessed. Zeolite is commonly associated with very pale brown, bioturbated, thin beds in Unit I. Given their euhedral crystal habit and disaggregated distribution, the zeolite likely formed in situ following bioturbation. Kastner (1986) states that these minerals are an alteration product of volcanic glasses. Consequently, the very pale brown sediment at Site U1365 likely corresponds to the deposition and subsequent alteration of volcanic ash falls. The absence of zeolite in the very pale sediment in Unit III is in accord with Stonecipher (1976), whose review of numerous DSDP reports found that phillipsite dissolves in many subseafloor sediments buried deeper than 50 mbsf. The porcellaneous and cherty intervals found in the lower half of the sediment column are only partially qualified by our investigations. APC coring in Hole U1365A fragmented the porcellanite and chert and created a well-mixed gravel of the pieces. Efforts to avoid the chert in Holes U1365B and U1365C by drilling through the cherty interval were largely successful. Although fragmented and disordered, observations of the composition and structure of the chert are possible in some of the larger pieces. For example, recovered fragments reveal very sharp contacts among dark brown silicified pelagic clay and light brown massive porcellanite. Relationships among porcellanite and chert are also shown in the fragments: porcellanite appears massive, whereas many chert fragments possess relict laminations. Unfortunately, the vertical position of the lithologic characteristics observed in the porcellanite and chert interval is unknown below the first meter of core because pieces of chert from anywhere in the ~20 m thick chert section could have sloughed into the hole between APC shots. Minor constituents of the stratigraphic section include fish teeth (ichthyoliths), silt-sized agglutinated foraminifers, quartz, transparent octahedral grains, fragmented radiolarians, and small silt-sized metallic spheres tentatively identified as micrometeorites. Examination for cosmic dust in the pelagic clay of this site was inspired by the results of the H.M.S. Challenger expedition, which discovered metallic microspherules in red deep-sea (~4300 mbsl) clay of the southern Pacific. These microspherules were identified as micrometeorites (Murray et al., 1891). The micrometeorites recovered by the Challenger expedition consist internally of metallic iron (90%) and nickel (10%), with thin a surficial crust of iron oxide (Jedwab, www.ulb.ac.be/sciences/cosmicdust.pdf). High concentrations of metallic microspherules of extraterrestrial origin also have been noted in other oceanic and land-based Cretaceous/Paleogene boundary sections across the globe (Smit and Romein, 1985; Ebihara and Miura, 1996; Grachev et al., 2008). Core catcher samples from Cores 329-U1365A-1H through 5H, 14H, and 24H; 329-U1365B-2H; and 329-U1365C-3H and 5H were examined for metallic micrometeorites. They were found in Samples 329-U1365B-2H-CC (13.8 mbsf), 329-U1365A-3H-CC (24.2 mbsf), 329-U1365C-3H-CC (25.3 mbsf), and 329-U1365A-4H-CC (34.7 mbsf). Micrometeorites are in greatest abundance around 20 mbsf (Fig. F14). No micrometeorites were found in core catchers below 34.7 mbsf. The metallic micrometeorites found at Site U1365 varied in diameter from 100 to 600 µm (Fig. F14A). Metallic particles were separated from the sediment by moving a small magnet under a small plastic dish containing the sediment sample. Magnetic grains (microspherules and metallic grains of different shapes) were then picked using a thin brush and mounted on paleontological slides for postexpedition study. IndurationSediment induration varies with lithology. Clay-rich sediment in the uppermost interval is very poorly indurated and becomes gradually firm in the first few meters below the sediment/water interface. At ~8 mbsf in Holes U1365A, U1365B, and U1365D, the sediment quickly becomes moderately indurated and the clay becomes slightly plastic. The induration of porcellanite is also variable. By definition, porcellanite is indurated sufficiently to resist deformation when handled (cf. Keene, 1975); however, some samples obtained from Hole U1365A were broken easily with fingers prior to sampling, whereas others required cutting with a trim saw. All chert samples were dense and tenacious. Some samples exhibited hairline fractures with alteration rims but remained competent and only broke when subjected to significant force. Metalliferous clay below the cherty interval in Holes U1365A–U1365C is very firm. In all holes, this interval contains 2–4 medium to thick (10–40 cm) beds of uniquely plastic dark reddish to greenish gray clay. Sedimentary structuresSedimentary structures at Site U1365 are subtle. Most of the clay intervals appear homogeneous. Beds (distinct sedimentary units whose thickness is >1 cm) are infrequent and found only in the lower half of the sediment column. Laminations (distinct units with thicknesses that are <1 cm) are slightly more common and appear more frequently in the lower parts of the upper and lower clay units. Mottling and burrowing of sediment at Site U1365 is pervasive but rarely intense. Burrows larger than 5 mm are very rare. However, given the overall fine-grained nature of the sediment, the disturbance of any primary bedding features by even minute organisms is easy to envision. For example, the foraminifers identified during biostratigraphic and thin section analyses are enormous relative to the average particle size of sediment at this site. Consequently, their motion on and through the mudline sediment could have played a significant role in disrupting the formation of laminations. The low variability in grain size (i.e., 2–200 µm) and dark color of the sediment potentially mask many bedding features. Several laminations formed in light-colored clay were observed to terminate or create wispy shapes that indicate moving water (possibly in the influence of ocean floor currents). Two deformation styles provide insight to postdepositional physical processes operating on sediment at Site U1365:
Coring may have induced artificial structure in Unit III. The dark reddish to dark greenish gray clay is sticky, and twisting of APC core liner during coring could cause stresses that deformed the sediment in this apparently compressional style. Interhole correlationStratigraphic correlation among the four APC holes at Site U1365 is straightforward (Fig. F9). The boundary between Subunits IA and IB is defined by the distinctive darkening of color associated with the underlying subunit. The precise depth of the contact is not certain in all holes, however, because whole-round sampling removed some of the Subunit IA/IB contacts in Holes U1365B and U1365D and the interval was not recovered in Hole U1365C. The Unit I/II contact is clearly identifiable in Holes U1365A and U1365B where pieces of the chert from Unit II were caught in the core catchers of Cores 5H of both holes. The position of the contact in Hole U1365C is less clear. Penetration of Core 329-U1365C-5H stopped abruptly at 37.61 mbsf in a portion of Subunit IB layered with porcellanite. Thereafter, drilling commenced, as it was assumed the top of the chert was reached. Accounting for differences in seafloor depth, the position of the Unit I/II contact in Hole U1365C is 5 m higher than the contact between Units I and II in Hole U1365A. If 37.61 mbsf is genuinely the base of Subunit IB in Hole U1365C, the basal porcellanite zone in that hole would be approximately one-half its thickness in Hole U1365A. Although this is certainly possible, it is counterintuitive to think that such large vertical differences could develop in the middle of a sedimentary succession in this deep, distal location. An alternative explanation is that the contact between Units I and II is lower than indicated by the short APC shot and that a short section of Subunit IB was bypassed in Hole U1365C. Drilling operations also impacted the correlation of contacts between Units II and III. Although the contact is clearly expressed in Hole U1365A, porcellanite and chert gravel cover poorly indurated sediment in core tops of Unit III in Holes U1365B and U1365C. Because this sediment is likely slough and fill, the exact location of the contact between Units II and III remains in doubt. Basement rock (basalt and basalt alteration products) provides the basal points of correlation among Site U1365 holes. APC coring in Holes U1365A–U1365C returned samples of the basement from all holes at depths that were within 2 m of each other when corrected for the different seafloor elevations of each hole. |
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