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doi:10.2204/iodp.proc.329.107.2011 LithostratigraphySediment at Site U1369 consists of ~16 m of zeolitic metalliferous clay. The principal components of the clay are phillipsite, red-brown to yellow-brown semiopaque oxide (RSO), and clay (see Site U1369 smear slides in “Core descriptions”; Fig. F7). The strata of the three holes at Site U1369 are divided into two lithologic units based on RSO abundance and size of phillipsite crystals (Fig. F8). The average RSO content of Unit I is 33%, and most of its phillipsite crystals are <10 µm in length. In the underlying Unit II, RSO concentration is ~42% and phillipsite crystals are frequently >50 µm long. Three manganese nodules were recovered from the mudline of Holes U1369B–U1369D. One additional nodule was found near the bottom of Unit II in Hole U1369B (Fig. F9A). A thin (~10 cm) layer of vitric sand/breccia was recovered from the base of Hole U1369E. Micro- and nannofossils are almost completely absent; only rare fish teeth were observed in the sediment. The overall texture of the sediment at Site U1369 is massive and appears to be the result of thorough bioturbation. Planolites burrows, increasing with depth, are outlined clearly in sediment that contains elevated abundances of phillipsite. Lithostratigraphic correlation among Site U1369 holes shows that sediment thickness and composition remain fairly uniform (Fig. F8). The pelagic sediment rests on vitric and altered basalt, which was sampled in small fragments in Holes U1369B, U1369C, and U1369E. See “Igneous lithostratigraphy, petrology, alteration, and structural geology” for more details regarding the basaltic rock recovered at Site U1369. Description of unitsUnit I
The overall color of the clay is brown (7.5YR 5/3) (Fig. F9B). 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. The manganese nodules recovered in Holes U1369B and U1369D are also black (2.5Y 2.5/1). A claystone recovered from interval 329-U1369E-1H-2, 115–117 cm, is very pale brown. Smear slide analyses identify phillipsite, RSO, and clay as the dominant sedimentary components of Unit I (Fig. F7). Phillipsite abundance varies between 38% and 63%. The higher modal abundances are associated with pale brown (10YR 8/4) and brown (7.5YR 5/4) ellipses as thick as 4 mm (Fig. F9B). Phillipsite crystals are very small (<10 µm) but euhedral (Fig. F10A). RSO grains are rounded to irregularly shaped, range in size from several micrometers to 50 µm, and constitute 24%–38% of the sediment. Clay abundance is generally low (~13%). Minerals identified by X-ray diffraction (XRD) analyses of the upper portion of clay in Unit I are smectite, illite, and chlorite (Fig. F11A). The characteristic XRD pattern of chlorite was not present in a second sample drawn from the gradational transition zone between Units I and II. Both samples, however, produced XRD patterns consistent with quartz and opal CT. Pebble- and cobble-sized manganese nodules are present at the top of Unit I in Cores 1H from Holes U1369B–U1369E. Approximately 17 g (dry mass) of Sample 329-U1369-1H-CC, 8–10 cm (the “PAL” biostratigraphy sample), was washed over 38 and 63 µm sieves to identify microfossil type and abundance. Only several fish teeth were observed in the remaining debris, and no calcareous or siliceous microfossils were found. Unit I sediment changes gradually from very poorly consolidated near the mudline to poorly consolidated at the base of the subunit. A very small number of highly indurated grains resisted grinding during preparation of the XRD samples. The individual grains were easily separated from the clay matrix, crushed, and remixed thoroughly with the remaining sample prior to XRD analysis. The structure in Unit I is massive with occasional burrows and rare very pale brown laminations. The burrows are Planolites associated with horizontal traces formed in the seafloor transition layer (5–8 cm) (Ekdale et al., 1984). A thoroughly bored hardground of very pale brown sediment was recovered from interval 329-U1369E-1H-2, 115–117 cm (Fig. F9C). The lower contact with Unit II is gradational, occurring through a 20–80 cm vertical length. Unit II
Clay in this unit is dark brown to very dark brown (7.5YR 3/4, 3/3, 2.5/3) (Fig. F9D). 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 associated with the edges of the rings. A manganese nodule recovered from the lower part of Hole U1369B is black (2.5Y 2.5/1). The lowermost 13 cm of Hole U1369E contains a distinctive sand/breccia of olive-yellow (5Y 6/8) angular grains with lesser amounts of white and red (2.5YR 4/8) interspersed clay. Smear slide analyses identify clay, RSO, and phillipsite as the dominant sedimentary components of Unit II (Fig. F7). RSO grains are rounded to irregularly shaped, range in size from <10 to 50 µm, and constitute an average of 42% of the sediment. Phillipsite abundance varies between 35% and 70%, with an approximate modal average of 40%. Many euhedral phillipsite crystals are large (>50 µm) (Fig. F10B). Clay abundance varies between 9% and 15%. The XRD peaks resulting from analysis of Sample 329-U1369B-2H-4, 80–82 cm, are representative of phillipsite, smectite, illite, chlorite, quartz, and opal CT (Fig. F11B). The olive-yellow sand/breccia in the lowermost 13 cm of Hole U1369E consists of angular grains of altered glass and clay. We refer to the coarse sand layer as a sand/breccia because its particles are particularly angular, but the grain size is considerably finer than typical breccias. Granules in the sand/breccia are translucent and possess concentric color bands similar to those associated with agates. However, XRD analyses of the sand/breccia indicates the granules are glass (i.e., incapable of producing an XRD peak characteristic of crystalline substances; see Fig. F11B). The XRD pattern of the sand/breccia’s clay fraction appears to be a mixture of phillipsite and illite/smectite. Consolidation of Unit II is uniformly firm. Sediment in Sections 329-U1369B-2H-7, 3H-1, 3H-2, and 3H-CC are flow-in. The sand/breccia at the base of Hole U1369E contains loose grains and clay that are easily disaggregated. Unit II structure is massive with increasingly common burrows in the 0.5–1 m of clay overlying its lower contact. The majority of burrows identified in Holes U1369B, U1369C, and U1369E are Planolites. Section 329-U1369E-2H-5 also contains wide (1 cm), highly vertical traces indicative of Thalassinoidies. The transition from clay to sand/breccia in the lowermost part of Hole U1369E is extremely abrupt and steeply (~75°) dipping (Fig. F9E). This contact is described more fully in the following section. Sediment/Basalt contactWe attempted to sample the sediment/basalt interface in each hole. Three small pieces of vitric basalt were recovered in Section 329-U1369B-3H-CC. See “Igneous lithostratigraphy, petrology, alteration, and structural geology” for a description of the basalt. Sediment surrounding the basaltic fragments was thoroughly disturbed. The soupy, very dark brown clayey sediment continued for a full 2 m above the interval containing the basaltic fragments. The first consolidated sediment overlying the interval of fragmented basaltic rock (interval 329-U1369B-3H-1, 15–19 cm) contains a manganese nodule. The sediment immediately overlying the nodule is dark brown zeolitic metalliferous pelagic clay. Fragments of altered basalt were recovered in Section 329-U1369C-2H-CC. The sediment in the core catcher was flow-in. The first sediment overlying the interval containing the basaltic fragments and not removed by whole-round sampling is interval 329-U1369C-2H-7, 51–53 cm. The sediment is massive dark brown zeolitic metalliferous pelagic clay. The lowermost interval of Hole U1369E contained two indicators of the sediment/basalt interface: (1) a chip of altered basalt was extracted from the lowermost part of the olive-yellow sand/breccia found in Sections 329-U1369E-2H-6 and 2H-CC (see “Igneous lithostratigraphy, petrology, alteration, and structural geology” for a description of the basaltic fragment) and (2) the sand/breccia overlying the basaltic fragment consists of vitric grains that were probably derived from volcanism that preceded sediment accumulation. The contact between the sand/breccia and overlying clay is unaltered, very sharp, and the clay and vitric materials do not intermingle. Within several centimeters above the sand/breccia-to-clay contact, the clay assumes the character typical of the remaining parts of Unit II, very dark brown, zeolitic, metalliferous, and extensively burrowed. Interhole correlationLithologic units are correlated among holes at Site U1369 to facilitate the integration of physical properties, geochemical, and microbiological data. The stratigraphic correlation panel for Site U1369 is presented in Figure F8. Correlations shown in this figure are based on principal characteristics of the sediment: (1) variable RSO concentration and phillipsite crystal size occurs abruptly between 5.5 and 6.5 mbsf (Fig. F7) and (2) magnetic susceptibility, gamma ray attenuation (GRA), and natural gamma ray (NGR) physical properties logs exhibit sharp increases through the interval that corresponds to the lithologic Unit I/II boundary. Correlations show that strata at Site U1369 have uniform unit thickness and composition. |
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