| IODP Proceedings Volume contents Search | |||
 | |||
| Expedition reports Research results Supplementary material Drilling maps Expedition bibliography | |||
|
doi:10.2204/iodp.proc.334.106.2012 Lithostratigraphy and petrologySite U1381 was drilled to investigate the lithostratigraphy of the sedimentary sequence on top of the Cocos Ridge as well as the uppermost portions of the Ridge to better characterize the material input into the subduction zone. Coring in a single hole from 0 to 165.04 mbsf recovered sediment, sedimentary rocks, and igneous rocks. The uppermost part of Hole U1381A is composed of a predominantly monotonous sequence of silty clay to clay. In the interval between Cores 334-U1381A-2R and 6R (~3.5–50 mbsf), a sequence of light greenish gray hemipelagic silicic ooze with terrigenous input is present. Three widely dispersed tephra horizons are recognized in a 14 m interval spanning from 34 to 48 mbsf. Below 51.1 mbsf (Cores 334-U1381A-7R through 11R), a sequence of 26 tephra horizons is observed. This predominantly dark grayish to yellowish brown clayey unit is characterized by a greater abundance of calcareous ooze than the interval above 51.1 mbsf. The contact between the basement and the overlying sediment was recovered in the lowermost segment of Section 334-U1381A-11R-CC at ~95.44 mbsf. The actual contact is not preserved but is designated by the first occurrence of aphyric to phyric basalt. The lithostratigraphic record of the igneous units is incomplete because of lower recovery rates in Cores 334-U1381A-12R through 26R. The igneous section (Unit III) consists predominantly of a sequence of fine- to coarse-grained pillow basalts. These pillow basalts are sometimes separated by a set of partly altered chilled margins and between 161.10–161.37 mbsf by a thin (27 cm) layer of volcaniclastics. Description of unitsThe cores recovered from Hole U1381A (Cores 334-U1381A-2R through 26R) can be divided into three lithostratigraphic units (Fig. F2). The 95.57 m cover sequence basement is divided into two units on the basis of lithologic attributes. Unit I
Unit I consists mainly of light greenish gray soft clay sediment with minor layers of silty clay and three tephra layers (Fig. F3). Tephra layers range from 2 to 4 cm thick, show gradation, and are partly disturbed and obscured by the rotary drilling process. In general, Unit I is massive with minor changes in the proportions of clay and silt. Biogenic components, especially nannofossils and diatoms, are abundant throughout the unit. Foraminifers, spicules, and radiolarians are present in trace abundances. Most of Unit I was intensely disrupted by RCB-related drilling disturbance. Silt-sized grains in smear slides include feldspar, chert, chlorite, pyroxene, amphibole, opaque minerals, calcite, glauconite, fragments of radiolarians, foraminifers, sponge spicules, glass, and rare quartz. Unit II
Unit II consists mainly of dark grayish to yellowish brown, soft to hardened clay(stone) (Fig. F4). Toward the base, the sediment is partly silicified. Unit II is distinguished from Unit I by its abundant biogenic components, which is also reflected in the abrupt change in color. The sediment is >70% spicules, diatoms, radiolarians, and nannofossils. With depth, the abundance of clay and the calcareous component increase within Unit II. Abundant tephra horizons, ranging from 1 to 8 cm thick, are massive and soft, show gradation, and are partly disturbed and smeared by rotary drilling. One notable exception is a 33 cm thick silicified mafic tephra layer in interval 334-U1381A-7R-1, 92 cm, to 7R-2, 9 cm, that shows parallel and cross lamination. Biogenic components (sponge spicules, nannofossils, diatoms, and foraminifers) dominate the sediment. Serravallian age lithic (sediment > magmatic) clasts and glass shards are still abundant in the silt to sand size of the detritus in the smear slides. Components present in trace abundances include feldspar, pyroxene, amphibole, calcite, and opaque minerals. Unit III
This unit consists mainly of pillow basalts of variable grain size, crystallinity, and vesicularity (see “Core descriptions”) that are sometimes separated by chilled margins (Figs. F5, F6, F7, F8, F9). The basalt ranges from aphyric to sparsely or highly plagioclase-phyric or even glomerophyric. Plagioclase phenocrysts are euhedral to anhedral. Groundmass ranges from microcrystalline to fine- (<0.5 mm) or medium- (0.5–1.0 mm) grained, consisting of plagioclase and pyroxene. Vesicularity varies from almost nonvesicular to highly vesicular (>20%; Fig. F6). Vesicles have round, elongate, and irregular shapes, especially those containing white to light green clay minerals, or the vesicles are spherical (preferentially filled with black to dark clay). Adjacent to chilled margins, the basalt is preferentially aphyric to sparsely plagioclase-phyric. The groundmass has the smallest grain size close to the chilled margins and becomes coarser farther away from the chilled margins. Chilled margins locally preserve glass; most of the glass is altered. The chilled margins including the glass at the contact and the altered basalt are of variable thickness and color. Chilled margins are 1 to 5 cm thick and range in color from dark gray or dark brown to lighter colors (Fig. F5). Tephra layersA total of 29 tephra layers are recognized in the core recovered at Hole U1381A. These tephra layers are intercalated with background Unit I (3 tephras) and Unit II (26 tephras) sediment. Individual tephra layers range in thickness from 1 to 33 cm. Unconformable and/or inclined bedding is rare. In some cores, localized bioturbation is observed at the tops of the tephra layers. Some of the tephra layers have a sharp basal contact to underlying sediment and gradual transitional contact with overlying ash-bearing sediment and are well sorted. Drilling disturbance overprints the original structure of the tephra layers. As a result, we are unable to identify internal sedimentary structures within many of the tephra horizons. Some tephra layers are lithified (Fig. F10). Besides the greater abundance of tephra horizons in Unit II, no systematic features can be correlated with depth. Compositions of the 29 identified tephra layers are variable. Of these 29 layers, 18 are pinkish gray/brown and 11 are pinkish/greenish black. Dark black tephra beds account for ~38% of the total tephra bed assemblage in Hole U1381A. The tephras in Unit I have similar mineral assemblages to those observed in Holes U1378B, U1379C, and U1380A. In contrast, the tephras in Unit II have generally lower crystal content than in Unit I and the sites arcward of the trench. In Unit II, the transparent glass shards of the more evolved tephras show no visible signs of alteration. Grain size ranges from medium to coarse ash (as large as millimeter size). The mineral assemblages consist of rare plagioclase and traces of pyroxene. Dark gray mafic ash layers consist predominantly of very coarse, dark to light brown sideromelane glass shards and rare tachylitic particles. Most of the sideromelane glass shards have blocky shapes and are medium to poorly vesicular but also show tubular-like glass shards. The glass shards are well preserved without obvious alteration features. The mineral assemblages of the mafic tephras include rare plagioclase and trace pyroxene. Depositional environment and correlation to Sites U1378 and U1379The cover sequence recovered from Hole U1381A consists of a sequence of hemipelagic siliceous ooze with terrigenous input, presumably remobilized from the slope sediment sequence, overlying pelagic calcareous ooze (see also “Paleontology and biostratigraphy”). AlterationAlteration of basalt in Hole U1381A occurs along vesicles and veins, as well as in matrix, forming rims, bands, or networks of alteration zones. The vesicles are filled with green clay, sometimes with zeolite in the center, and accompanying aggregates of pyrite. The surrounding glass is altered to narrow (0.1–0.3 mm) black rims around the vesicles composed of iron oxyhydroxides and dark gray clay. Most of the veins are composed of green clay with minor amounts of calcite and pyrite aggregates. The vein wall and rims are altered to black. Apart from vesicles and veins, black alteration zones form irregular straight bands or irregular spots. In these black alteration zones, glass is completely altered, whereas groundmass plagioclase tends to survive. Some of plagioclase phenocrysts in these alteration zones are altered completely to green clay, forming green spots. In general, the intensity of alteration generally increases with depth. |
|||
