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doi:10.2204/iodp.proc.344.103.2013

Lithostratigraphy and petrology

During Expedition 334, Holes U1381A and U1381B were drilled to investigate the lithostratigraphy and pore water of the overlying sequence on top of the Cocos Ridge basement, as well as the uppermost portions of the adjacent Cocos Ridge. The Cocos Ridge was cored successfully with the RCB; however, the sedimentary section was associated with low recovery (between 42% and 54%). A goal of Expedition 344 was to fully recover the sedimentary succession by coring Hole U1381C to the sediment/​basement contact with the APC. Successful coring from 0 to 103.83 mbsf yielded a succession of sediment, sedimentary rocks, and a short interval of basaltic breccia in the lowermost part of the hole. Tephra layer abundance is generally high (84 tephras in total) but is concentrated in the lower part (~40 to ~100 mbsf) of the recovered sedimentary succession. This cored material was divided into five units (Fig. F3; Table T2).

The uppermost part of Hole U1381C is characterized by a predominantly monotonous sequence of hemipelagic silty clay to clay sediment. Unit I (0–55.93 mbsf) comprises a sequence of light greenish gray hemipelagic silty clay containing terrigenous material (lithic fragments, glass shards, and minerals), as well as common to abundant foraminifers and nannofossils. Tephra layers make up ~1.2% of Unit I and are distributed into 18 well-sorted discrete tephra horizons. The boundary between Units I and II at 55.93 mbsf (Section 344-U1381C-7H-1, 33 cm) represents a hiatus of 9–11 m.y. and separates sediments with ages between ~1.9 and ~13.53 Ma (see “Paleontology and biostratigraphy”).

Unit II (55.93–100.64 mbsf; Sections 344-U1381C-7H-1, 33 cm, to 11H-5, 104 cm) is a 44.71 m thick dark grayish to yellowish brown clayey interval composed of nannofossil-rich calcareous ooze and variable amounts of sponge spicules, foraminifers, and diatoms. Tephra layers (~6% of Unit II) are thickest and most frequent in the uppermost part of the unit. A total of 62 dark brown to gray and sometimes light gray tephra horizons are preserved as discrete layers or individual and/or layered ash pods.

Unit III is a 2.56 m thick interval between 100.64 and 103.20 mbsf (Sections 344-U1381C-11H-5, 104 cm, to 12H-1, 10 cm). This unit is a nannofossil calcareous ooze characterized by an abrupt decrease in the abundance of sponge spicules and foraminifers and an increase in clay content. Reworked clay clasts from the underlying Unit IV can be found in the lowermost 10 cm. Tephra layers and pods are rare (<1%) in this interval, and only four dark brown layers were recovered.

Unit IV is a 0.35 m thick interval between 103.20 and 103.55 mbsf (Sections 344-U1381C-12H-1, 10 cm, to 12H-CC, 16 cm) and consists of a dark brown sequence of well-consolidated clay and lithified claystone almost completely devoid of biogenic components. Unit V, cored to a total depth of 103.83 mbsf (Section 344-U1381C-13X-1, 0–33 cm), is a 33 cm thick basalt breccia.

Description of units

The cores recovered from Hole U1381C are divided into five lithostratigraphic units (Fig. F3; Table T2) that span the 103.55 m cover sequence above the basement, as well as the interface with the basaltic breccia below.

Unit I

• Interval: Sections 344-U1381C-1H-1, 0 cm, to 7H-1, 33 cm
• Thickness: 55.93 m
• Depth: 0–55.93 mbsf
• Age: early Pleistocene to recent
• Lithology: soft silty clay to clay sediment

Unit I consists mainly of massive, light greenish gray, soft (silty) clay sediment with minor changes in the proportions of clay and silt in the background sedimentation (Fig. F4). Biogenic components, especially nannofossils and diatoms, are abundant throughout the unit. Silt-sized grains in smear slides include common clastic components such as plagioclase, chert, chlorite, pyroxene, amphibole, opaque minerals, calcite, glauconite, glass, and rare quartz, as well as traces of biogenic fragments such as radiolarians, foraminifers, and sponge spicules. Eighteen well-sorted tephra layers ranging in thickness from 1 to 7 cm are normally graded and moderately disturbed and smeared out by drilling operations (Figs. F5, F6).

Unit II

• Interval: Sections 344-U1381C-7H-1, 33 cm, to 11H-5, 104 cm
• Thickness: 44.71 m
• Depth: 55.93–100.64 mbsf
• Age: early Miocene
• Lithology: nannofossil calcareous ooze with sponge spicules

Unit II mainly consists of dark grayish to yellowish brown, soft to hardened silty clayey calcareous ooze with abundant sponge spicules and varying amounts of foraminifers (Fig. F7). Unit II is distinguished from Unit I by its abundant biogenic content, which is also reflected in the abrupt change in color. The sediment contains >70% organic components such as spicules, diatoms, radiolarians, and nannofossils, with the abundance of clay and calcareous components increasing with depth. Mineral components that occur in trace amounts include feldspar, pyroxene, olivine, calcite, and opaque minerals. Sixty-two tephra horizons, ranging from 1 to 41 cm in thickness, are massive and soft, show normal gradation from medium sand to silt, and occur as discrete layers, individual ash pods, or ash pod layers (Figs. F8, F9). These horizons are well sorted, which is typical of marine tephras (e.g., Carey, 1997), and some are partially disturbed and smeared by bioturbation and drilling at their base. In general, the abundance and thickness of these predominantly mafic tephra layers decreases with depth.

Unit III

• Interval: Sections 344-U1381C-11H-5, 104 cm, to 12H-1, 10 cm
• Thickness: 2.56 m
• Depth: 100.64–103.20 mbsf
• Age: middle Miocene
• Lithology: nannofossil ooze

The boundary between Units II and III displays a clear change from dark grayish to yellowish brown sediment to a dark brown color and finer grained nannofossil-dominated clay (Fig. F10). Minor matrix components include rare glass shards and feldspar crystals. The lowermost part of this unit (interval 344-U1381C-12H-1, 0–10 cm) contains laminations ~5 mm in scale, as well as abundant well-rounded dark greenish black claystone clasts that are most likely reworked material from Unit IV, below (Fig. F11). Four tephra layers were identified in this unit.

Unit IV

• Interval: Sections 344-U1381C-12H-1, 10 cm, to 12H-CC, 16 cm
• Thickness: 0.35 m
• Depth: 103.20–103.55 mbsf
• Age: middle Miocene
• Lithology: claystone

Unit IV consists of a dark greenish black, 35 cm thick sequence of very consolidated and partly lithified clay to claystone (Fig. F11). This unit contains only trace amounts of nannofossils and lacks any other biogenic components. The entire sedimentary sequence is highly disturbed and fractured by drilling.

Unit V

• Interval: Section 344-U1381C-13X-1, 0–33 cm
• Thickness: 0.33 m
• Depth: 103.50–103.83 mbsf
• Age: middle Miocene
• Lithology: basaltic breccia

Unit V consists of rubble with preserved matrix in the uppermost 5 cm, followed by poorly sorted brecciated basaltic fragments varying from 1 mm to 10 cm in size (Fig. F12). No obvious contact between Units IV and V was recovered.

Basaltic clasts and rubble throughout Unit V range from sparsely to highly plagioclase-clinopyroxene phyric, whereas textures may vary from variolitic to intersertal. Groundmass composition consists of microcrystalline plagioclase and pyroxene with accessory Fe-Ti oxides. Many fragments exhibit curved chilled margins with fresh to devitrified glassy rims. Approximately 10% of the groundmass is made up of euhedral- to anhedral-shaped plagioclase phenocrysts that range in size from 0.1 to 4 mm. Vesicles represent 3% of the recovered basaltic fragments and are typically irregular but well rounded. Vesicle abundance in larger clasts and rubble is greatest near the chilled margins.

Overall alteration of the basaltic groundmass is slight to moderate, with smectite replacing interstitial glass and partially corroding plagioclase and clinopyroxene phenocrysts. Black halos near the chilled margins and flanking veins are composed of secondary pyrite. Within the main rubble interval (344-U1381C-13X-1, 15–33 cm), larger fragments preserve narrow (<0.1 mm) veins of sulfides and smectite, whereas smaller fragments may be partially to completely covered by sulfides. Vesicles within the basaltic rubble are 50% filled by smectite and are typically flanked by narrow pyrite halos. The vein and vesicle mineralization sequence implied from thin section analyses suggests that sulfide mineralization predates smectite.

Matrix mineralogy observed from smear slides in the uppermost part of the unit (interval 344-U1381C-13X-1, 0–5 cm) includes undefined clay minerals, plagioclase, clinopyroxene, and pyrite, implying that the matrix is basaltic in origin, tentatively the result of extensive alteration of a lava flow collapse.

Tephra layers

A total of 84 tephra layers are recognized in Hole U1381C. These tephra layers are intercalated with sediment from background Unit I (18 tephras), Unit II (62 tephras), and Unit III (4 tephras). Individual tephra layers range in thickness from 1 to 41 cm. Unconformable and/or inclined bedding is rare. Most of the tephra layers are well sorted and have a sharp basal contact with the underlying sediment but a gradual transition with the overlying, ash-bearing sediment. Normal grading can often be seen in the tephra horizons and even in the large ash pods. Some tephra layers are lithified (Figs. F8, F9). In some cores, localized bioturbation is observed at the tephra layer boundaries (Fig. F6). Drilling disturbance rarely overprints the original structure of the tephra layers (Fig. F9). Aside from the greater abundance of tephra horizons in the upper part of Unit II, no systematic features were correlated with depth.

The compositions of the 84 identified tephra layers seem to be variable, as evident from the gray to pinkish (more felsic, silica rich; predominantly Unit I; e.g., Figs. F5, F6) and brownish black (more mafic, silica-poor; predominantly Unit II; e.g., Figs. F8, F9) colors. Dark black mafic tephra beds account for ~27% of the total tephra bed assemblage of Unit I in Hole U1381C but are more abundant in Unit II (~72%). Unit I tephras have mineral assemblages similar to those observed during Expedition 334 in Holes U1378B, U1379C, and U1380A.

Light-colored felsic tephra layers contain mainly transparent glass shards that are mostly fresh with few signs of alteration, as well as pumiceous clasts. Devitrification structures within glass shards increase with depth, reflecting increasing alteration with depth. Grain size ranges from very fine to coarse ash (up to millimeter sized). The mineral assemblages consist of plagioclase, pyroxene, hornblende, and biotite. Plagioclase is the dominant phenocryst phase, but some tephras are dominated by amphibole and biotite.

Dark gray mafic tephra layers consist predominantly of very coarse (up to 3 mm), dark to light brown sideromelane glass shards and rare tachylitic particles. Most of the glass shards have blocky shapes and are medium to poorly vesicular but also show tubular-like glass shards. The mineral assemblages of the black mafic tephras in Unit II include rare plagioclase and traces of pyroxene and olivine, but in general, the number of phenocrysts is much lower than in tephras found in Unit I. Glass shards are increasingly affected by alteration in the deeper part of the hole.

X-ray diffraction analysis

Preliminary X-ray diffraction analysis of sediment samples from Hole U1381C (Fig. F13) suggests there is little variation in composition within each lithostratigraphic unit (Fig. F14).

X-ray diffractograms of Unit I indicate that the major mineral components are phyllosilicates, including chlorite and smectite, calcite, Ca- and Na-plagioclase, and quartz (Fig. F13). The relative abundances of major minerals in Unit I are constant overall, with a slight decrease of quartz and associated increase of calcite toward the base (Fig. F14). Amphibole (hornblende), pyroxene (diopside), zeolites (stilbite and laumontite), and pyrite peaks are also present. Biotite is present in some samples in the lower part of the unit (Cores 344-U1381C-4H and 6H), sometimes in large amounts.

Variation in mineral composition is most strongly noted in the sandy sediment collected from Unit I and the upper section of Unit II, where biogenic fragments are more common (Fig. F14). Samples from Unit II produced spectra dominated by calcite and Mg-rich calcite with pyrite. Common Ca- and Na-plagioclase and rare quartz are also present throughout Unit II (Fig. F13).

X-ray diffractograms of samples from Unit III (Figs. F13, F14) indicate that the major mineral components are calcite, zeolite (Ca-heulandite), and smectite. Plagioclase and pyrite are present as minor phases. The vein sample in Section 344-U1381C-11H-7 is composed of zeolite and smectite.

Unit IV comprises a mixture of smectite (most likely saponite), pyrite, and minor plagioclase (Fig. F14). Sphalerite, calcite, and anhydrite are present as minor occasional phases.

Depositional environment and correlation to Hole U1381A

The cover sequence recovered from Hole U1381C consists of a sequence of hemipelagic silty clay with terrigenous input, presumably remobilized from the slope sediment sequence, overlying sponge spicule–rich nannofossil calcareous ooze, pelagic calcareous ooze, and pelagic claystone. The lithologic occurrence and the depth ranges of the unit boundaries are similar to Hole U1381A, drilled during Expedition 334. The conspicuous, large hiatus between hemipelagic Unit I and pelagic Unit II (see “Paleontology and biostratigraphy”) was also found at Site 1242 (Ocean Drilling Program [ODP] Leg 202) but differs in boundary ages (Pliocene to middle Miocene) and the absence of the characteristic mafic tephra layers found in Unit II.

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