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doi:10.2204/iodp.proc.329.108.2011 LithostratigraphyThe sediment at Site U1370 is ~70 m thick. The dominant lithology is zeolitic metalliferous pelagic clay. The strata also contain a relatively short (30 cm to 2.9 m) interval of very pale brown to pale yellow nannofossil ooze that divides the clay into upper and lower units. The principal components of the clay are red-brown to yellow-brown semiopaque oxide (RSO), phillipsite, and smectite (see Site U1370 smear slides in “Core descriptions”; Fig. F7). Lithologic Unit I lies between the sediment/water interface and the top of the nannofossil ooze at ~61 mbsf. Modal abundances of RSO, phillipsite, and clay are 36%, 29%, and 27%, respectively. Based on smear slide analyses, the ooze (Unit II) contains 96% coccolithophores, 3% phillipsite, and 1% clay. The lower clay (Unit III) is thin; it contains 88% RSO, 12% clay, and 0% phillipsite and directly overlays the basaltic basement. Volcanic glass is locally abundant (~43%) in Unit I; however, its overall abundance is only 7% and it is completely absent in Units II and III. Manganese nodules were not recovered in mudline cores. A large, fragmented nodule was recovered in Hole U1370D at 10 mbsf, and fragments of manganese-encrusted hardground were recovered in Hole U1370F at 52 mbsf. The overall structure of the dark brown pelagic clay and pale yellow nannofossil ooze at Site U1370 is massive, although occasional laminations and thin beds are visible in the lower half of Unit I. Planolites (horizontal) burrows are faintly visible in most of the mottled clay and are occasionally highlighted by very pale ellipses in the upper part of Unit I. Trichichnus (vertical) burrows blend the upper and lower contacts of the pale yellow nannofossil ooze and the over- and underlying metalliferous clay. Lithostratigraphic correlation among Site U1370 holes shows that sediment thickness and composition are uniform (Fig. F8). The pelagic sediment rests on altered basalt that was recovered in small fragments in Cores 329-U1370D-8H (~68 mbsf) and 329-U1370F-8H (~65 mbsf). Description of unitsUnit ISubunit IA
The overall color of the clay is brown (7.5YR 5/3) (Fig. F9A). Circular and elliptical rings, 1–2 mm thick with diameters between 1 and 3 cm, are very pale brown (10YR 8/4) with occasional black (2.5Y 2.5/1) grains located along the inner and outer rims of the rings. In the intervals with a large number of rings, the overall color is a slightly lighter shade of dark brown (10YR 3/3). The manganese nodule and hardground recovered in Hole U1370D and U1370F, respectively, are black (2.5/N) (Fig. F9B). Smear slide analyses identify RSO, phillipsite, and clay as the dominant sedimentary components of Subunit IA (Fig. F10A). The lower contact of the subunit is based on a gradational color change that corresponds to modal changes in phillipsite abundance. Average phillipsite abundance is 26% and changes very little throughout the upper 45 m of Subunit IA (Fig. F7). Phillipsite abundance decreases sharply through the lower 8 m of the subunit. Clay occurs in a wide range of abundances, from 13% to 43%, with its highest average concentration in the middle third of Subunit IA. Minerals identified by X-ray diffraction (XRD) analyses are smectite and illite. Other minerals identified by XRD are phillipsite, quartz, cristobalite, tridymite, and plagioclase (Fig. F11A). The cristobalite and tridymite could not be confirmed visually in smear slides and may exist as tiny crystals in the clay. The manganese nodule found in Hole U1370D at 10 mbsf filled the core liner completely and was apparently trimmed by coring. When sawn in half, the cross-sections showed multiple concentric rings of increasing diameter that coalesced to form the single, large nodule. The exterior surface of the closely spaced manganese-encrusted sediment (i.e., hardground) found at 52.5 mbsf in Hole U1370F contained warty surfaces and a friable exterior rind. The mudline sample from Hole U1370B and two other samples deeper in this unit (Samples 329-U1370B-1H-CC and 2H-CC [residue from catwalk processing]) were scanned for fossils and found to be barren except for rare fish teeth. Subunit IA sediment changes gradually from very poorly consolidated near the mudline to poorly consolidated at the base of the subunit. The manganese nodule and hardground in Holes U1370D and U1370F, respectively, are well consolidated but brittle. The structure of Subunit IA is massive with occasional burrows and thin bedding of various thickness (3 mm to 2 cm), color (gray to brown), and sharpness (diffuse to sharp). The majority of the burrows are Planolites associated with horizontal traces formed in the seafloor transition layer (5–8 cm; Ekdale et al., 1984). Occasional vertical burrows of unknown ichnofacies association appear in Subunit IA below 4 mbsf. Bedding features are not uniformly distributed. For example, layering is more apparent in the lower subunit intervals. The lower contact with Subunit IB occurs at a core break in Holes U1370D and U1370F. In Hole U1370E, the boundary was likely in the bottom of Core 329-U1370E-6H (Fig. F8); however, the bottom of that core was lost during core recovery. Furthermore, Cores 329-U1370E-7H and 8H consisted of 100% disturbed (fall-in and flow-in) sediment. Thus, the only estimate of the position of the Subunit IA/IB boundary in Hole U1370E that we can make is that it exists below Core 329-U1370E-6H and at a depth of >51.6 mbsf. Subunit IB
The overall color of the clay is dark brown (7.5YR 3/2), gradually changing to black (7.5YR 2.5/1) in the lower meter of the interval (Fig. F9D). A very small portion of Subunit IB contains elliptical burrow traces with outlines that are very pale brown (10YR 8/4). Claystone pieces and matrix recovered from interval 329-U1370D-7H-3, 126–128 cm (Fig. F9C), are olive-brown (2.5Y 4/4). In Hole U1370F, the same layer lacks claystone pieces but includes planar burrow traces along its lower contact that are white (10YR 8/1) (Fig. F12). The interval immediately overlying the lower contact (~3 cm thick) is very dark brown (7.5YR 2.5/2). Smear slide analyses identify RSO, phillipsite, and clay as the dominant sedimentary components of Subunit IB (Fig. F7). Although RSO concentration is sharply lower than that found in the lower 5 m of Subunit IA, the overall abundance increases to 40% (an increase of 16%). Phillipsite abundance varies between 18% and 59% with an average modal concentration of 38%. Throughout the unit, the phillipsite crystals range in size from <10 to 60 µm (Fig. F10B). Clay abundance drops from an average of 30% in Subunit IA to 16% in Subunit IB. XRD analysis of Sample 329-U1370D-7H-2, 143–144 cm, a representative sample from the middle of Subunit IB, shows that clay mineralogy is illite-smectite (Fig. F11A). Several peaks in the diffractogram are unresolved but could result from the very small amounts of plagioclase and quartz noted during petrographic inspection of smear slides or weakly crystalline iron and manganese hydroxides associated with the RSO. The only fossils noted in smear slide analyses of Subunit IB are coccolithophores in the lowermost (3 cm) interval immediately overlying Unit II. Their abundance is roughly 5% of the total sediment volume. Subunit IB is firm clay. The claystone hardground located just below the interval’s midpoint is highly indurated. Subunit IB is ~8.5 m thick. The upper 1 m is massive with faint mottling. Occasionally isolated burrows (2 cm maximum diameter) are visible in the middle of the very dark brown sediment. The middle of the interval also contains bioturbation that forms 1–3 cm thick brown and dark gray horizontal layers. There are no burrows in the 70 cm overlying Unit II. A claystone hardground 2 cm thick and consisting of small clay pebbles residing in a coarse matrix was observed in intervals 329-U1370D-7H-3, 126–128 cm, and 329-U1370F-7H-3, 79–85 cm (Fig. F9C). Upper and lower contacts of the bed are irregular and bioturbated. In Hole U1370F, the lower contact of the hardground contains thin, white, hexagonal Paleodictyon burrow traces (Fig. F12). The lower contact of Subunit IB (with Unit II) has three notable attributes:
Unit II
The overall color of the ooze is very pale brown (10YR 8/4) to pale yellow (2.5Y 8/4–8/6) (Fig. F9D). Darker shades of pale brown and pale yellow are associated with vertical burrows that lie in close proximity to the overlying (black) zeolitic metalliferous pelagic clay. Two nearly white (10YR 8/1) sediment layers (6 cm and 1 mm thick) lie 1 m and 1.1 m below the upper unit contact in Hole U1370D (Fig. F9E). One of these layers was observed in photographs of the ends of two sealed whole-round samples taken for microbiology studies in Hole U1370F. Smear slide analyses show that Unit II is nannofossil ooze (Fig. F10C). The unit matrix is composed of disaggregated coccolithophorid tests. The ooze is nearly pure calcite containing small amounts of phillipsite (3%) and RSO (1%) only where bioturbation has mixed Unit II with Units I and III (Figs. F7, F11B). There are two white intervals that are 100% coccolithophores, coccolithophorid debris, and foraminifers. Micropaleontological analyses of wet-sieved samples show that the sediment fraction >38 µm is composed mainly of planktonic and benthic foraminifers of early Paleocene age (planktonic foraminifer Zone P1). The foraminifers found in Unit II are described fully in “Paleontology and biostratigraphy.” Unit II sediment is moderately consolidated and was recovered in Holes U1370D and U1370F. The stratigraphically equivalent interval in Hole U1370E recovered only fall-in and flow-in sediment. In Hole U1370D, the unit spans Cores 329-U1370D-7H and 8H. Unit II occupies the lowermost 140 cm of Core 7H (Sections 329-U1370D-7H-6, 7H-7, and 7H-CC). Core 329-U1370D-8H includes 12 cm of undisturbed ooze in Sections 329-U1370D-8H-1 and 8H-2 and 1.4 m of fall-in in the upper part of Section 8H-1. Thus, the true stratigraphic thickness of Unit II is likely 1.52 m, although as much as 2.9 m of Unit II was recovered in Hole U1370D. In Hole U1370F, the entire interval was removed by microbiological whole-round sampling. After observing the whole-round ends and photographs of whole-round ends, we conclude that the total thickness of Unit II in Hole U1370F is between 30 and 40 cm. Because cross-sections of the whole-round samples could not be observed, the cause of the observed difference in stratigraphic thickness cannot be ascertained and the description of structure that follows is based exclusively on observations from Hole U1370D. Unit II is thoroughly bioturbated. Trichichnus (vertical) burrows are visible throughout the unit. The maximum burrow width is 6 mm and the length of an individual burrow ranges from 15 cm within the unit to 35–50 cm into the underlying clay (Fig. F9F). Two intervals within Unit II display white horizontal layering with clearly visible, small-scale (1–3 cm) bioturbation effects. The upper layer is 6 cm thick and the lower layer is ~1 mm thick. The lower contact of Unit II contains two tiers of bioturbation. The upper tier comprises 10 cm of mixed ooze and clay that is completely filled with short, overlapping vertical burrows. In the lower tier of burrowing, distinct Trichichnus burrows extend as far as 50 cm into the underlying metalliferous clay. Unit III
The color of the clay is very dark brown (7.5YR 2.5/3) to black (7.5YR 2.5/1) (Fig. F9F). There are no visible color changes. Smear slide analyses show that RSO and clay are the two principal components of Unit III. RSO is dominant, comprising 88% of the unit (Fig. F10D). Although RSO grains come in a variety of sizes from <10 to 50 µm, most are ~10 µm with relatively few large semiopaque grains. The relative purity of the interval allows XRD results to partially resolve the diffraction pattern created by RSO. As shown in Figure F11B, the broad hump between 20.5° and 22°2θ is centered on the peak locations for goethite (Fe+3O[OH]; ICDD card 00029-0713) and ramsdellite (MnO2; ICDD card 00-042-1316). These results do not, however, mandate the occurrence of goethite and ramsdellite within the RSO component. Rather, they support previous work that suggests RSO is comprised of iron and manganese oxyhydroxides in poorly crystalline to amorphous solids (Heath and Dymond, 1977; Kastner, 1986). The clay is uniformly firm. Flow-in occurred in Holes U1370D–U1370F. Flow-in was easily identified because it contained grains and clasts of green altered basalt. There are no visible structures in any of the recovered portions of Unit III other than the Trichichnus burrows described previously. The unit’s lower contact was not recovered in any of the holes (i.e., the recovery at the bottom of each hole was flow-in). Although the flow-in contained metalliferous clay from Unit III and basalt fragments from the underlying formation, the actual contact was not recovered. Interhole correlationLithologic units are correlated among holes at Site U1370 to facilitate the integration of physical property, geochemical, and microbiological data. The stratigraphic correlation panel for Site U1370 is presented in Figure F8. Correlations shown in this figure are based on principal characteristics of the sediment, including
Lithostratigraphic correlation shows the strata at Site U1370 are not uniform. The most significant difference among the holes is the thickness of Unit II. Although coring disturbances make correlation difficult, Unit II thickness could differ by as much as 50% between Holes U1370D and U1370F. Lithology of Unit II in Hole U1370F could not be petrographically inspected or analyzed by XRD, but based on visual inspection of the sediment from Hole U1370F through core liner end caps, the unit appears to have similar composition in Holes U1370D and U1370F. Manganese nodules and hardgrounds are also unevenly distributed with depth among the three holes at Site U1370. However, Units I and III appear to be similar in both thickness and composition. |
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