Figure F28. Representative minerals and textures in veins and areas of alteration, igneous Subunit 2a. A. Dolomite (Dol), chlorite (Chl), quartz, and clay minerals in a vein crosscutting altered plagioclase phyric basalt (PPL). B. Amygdule filled with a brown zeolite mineral in altered basalt; amygdule has a rim rich in Fe hydroxide minerals (PPL). C. Matrix in a basaltic breccia with dolomite rhombs and Fe hydroxide minerals and minor clay minerals, calcite, epidote, and zeolite minerals (PPL). D. Nodules of radiating calcite (Cal) crystals with minor dolomite, clay minerals, quartz, and epidote (PPL). E. Vein (Type 1) with calcite, quartz (Qtz), and pyrite (Py) in altered basalt with chlorite (XPL). F. Matrix in a breccia at the base of subunit (XPL); dolomite and/or siderite (Dol/Sd) with quartz, pyrite, and clay minerals are predominant in the breccia matrix.

<figure><img src="image_310.png" /></figure>

be visible in thin section, as well as some Fe-Ti oxides, but no other primary magmatic minerals are present. The green color of the basalt is most likely due to the presence of epidote, green zeolite minerals, and green clay minerals in the upper sections and chlorite in the lower sections (Figures F26, F28, F31). Other common alteration minerals are calcite, dolomite, quartz, chalcedony, and Fe (hydr)oxide (Figures F26, F28). Pyrite is pervasive and is found in the fractured basalt, in the matrix and clasts of the basalt breccia, in pillows, as vesicle filling, and in veins (Figures F26, F31). In addition,

minor amounts of chalcopyrite, sphalerite, and covellite are found in the groundmass and veins in Cores 23R through 32R.

There are at least three vein types. Type 1 is white to green to black in color, with cryptocrystalline silica, epidote, Fe (hydr)oxide, carbonates (dolomite/siderite), and traces of pyrite (e.g., intervals 10R-1, 68–85 cm, and 13R-4, 90–100 cm; Figures F26B, F26C, F28E). Some veins have less abundant quartz and are green to grayish green, whereas others have red jasper. Type 2 is pyrite rich with pistachio green and dark green minerals, probably epidote and zeolite minerals and quartz (e.g., interval 13R-3, 110–120 cm; Figure F26D–F26F). Type 3 veins are rich in calcite with minor epidote and pyrite (e.g., interval 19R-2, 50–68 cm) (Figures F26A–F26C, F31).

## Bulk mineralogy

The bulk mineralogy results of XRD analyses on igneous samples from Site U1502 are listed in Table T3 and shown in Figures F31 and F32. Igneous Unit 2 basalts are composed mainly of plagioclase (mean = 78%), with minor amounts of augite (mean = 11%), quartz (mean = 6%), and clay minerals (smectite; mean = ~1%) (Figure F32; Table T3). Pyrite is found both in veins and as a disseminated phase in numerous basalt intervals within igneous Unit 2, and it is a common accessory phase in many of the basalt XRD analyses. Differences in additional mineral phases are found between Subunits 2a and 2b. Chlorite is not common in Subunit 2a basalts, and when detected, it is <1%. In contrast, most basalts in Subunit 2b contain chlorite, and its abundance increases with depth, reaching 5% in Section 368-U1502B-37R-4. Many basalt samples in Subunit 2a contain as much as 3% calcite and/or dolomite, and two samples (intervals 7R-1, 23–25 cm, and 7R-2, 122–124 cm) are notable for having 9%–12% siderite. Kaolinite is found in the lowermost 7 m of Subunit 2a and throughout Subunit 2b, where its abundance increases downhole from ~1% to 8%. XRD samples taken from highly fragmented (drilling-disturbed) intervals (e.g., 10R-2, 125–127 cm; 10R-3, 16–18 cm; and 11R-2, 4–6 cm) generally have lower amounts of plagioclase and higher amounts of quartz, pyrite, and siderite ± dolomite.

XRD analyses were made to confirm the identity of epidote (olive green) and chlorite (dark green) in veins from intervals 16R-2, 15–19 cm, and 16R-1, 93–96 cm, respectively (see Figure F31). Samples from interval 16R-2, 15–19 cm, contain 21% epidote (with 8% chlorite because it was difficult to hand pick a clean sample of the epidote), and samples from interval 16R-2, 15–19 cm, contain 13% chlorite plus feldspar and clay minerals.

## Chemical composition (pXRF and XRF core scanning)

Compositional analyses of the igneous lithologic Unit 2 basalts, as well as veins and clasts, were made directly on the archive half of the core using the handheld pXRF (see **Igneous and metamorphic petrology** in the Expedition 367/368 methods chapter [Sun et al., 2018a]). The average pXRF sample spacing was about every 1 m. Additional representative samples were also taken for major and trace element analysis by ICP-AES (see **Geochemistry**). The pXRF data are shown in Figure F33 and Table T4. Subunit 2a has lower MgO (<4 wt%) and MnO (<0.1 wt%) contents than Subunit 2b (MgO = >4 wt%; MnO = >0.1 wt%), and on average it also has lower Fe₂O₃ (4.8 ± 3.1 wt% versus 8.5 ± 2.6 wt%). This difference is interpreted as a secondary feature related to the pervasive alteration of Subunit 2a because there are no significant differences in abundance,
Figure F29. Pillow lavas, igneous Subunit 2b. A. Partial pillow showing altered rim. Formerly glassy areas in the rim are altered to an assemblage of dominantly chlorite, kaolinite, and smectite (see Figure F31 for XRD pattern of this material). Altered rim is plagioclasephyric, and pipe vesicles in whitish altered basalt close to the rim run perpendicular to it. Detail of rim is shown in C. B. Two partial pillows. Rims are altered to green minerals, including chlorite, clay, and zeolite minerals cut by veins of calcite and epidote. Pipe vesicles are perpendicular to pillow rims. Light green alteration minerals occur in a zone parallel to the pillow rim and as a halo to an epidote-rich vein. C. Pillow rim displayed in A with highly altered plagioclase phenocrysts (now albite, sericite, epidote, and calcite) in an altered groundmass of quenched plagioclase, calcite, clay minerals, epidote, pyrite, and zeolite minerals (PPL). D. Pillow rim with highly altered plagioclase phenocrysts (now albite, sericite, and calcite) in a formerly glassy groundmass now altered to clay minerals, zeolite minerals, epidote, pyrite, and calcite and plagioclase microlites (PPL). E. Pillow rim with highly altered plagioclase microlites in a formerly glassy groundmass now altered to epidote, clay minerals, zeolite minerals, pyrite, and calcite (PPL). Relict perlitic cracking is highlighted by the texture of the alteration minerals. F. Pillow rim with highly altered plagioclase phenocrysts and microlites (quench textured) in a formerly glassy groundmass now altered to epidote, clay minerals, zeolite minerals, pyrite, and calcite (XPL). Relict perlitic cracking is highlighted by the texture of the alteration minerals.

368-U1502B-18R-2, 90-102 cm

<figure><img src="image_311.png" /></figure>

368-U1502B-19R-1, 115-137 cm

<figure><img src="image_312.png" /></figure>

368-U1502B-18R-2, 91-94 cm

<figure><img src="image_313.png" /></figure>

368-U1502B-19R-1, 102-105 cm

<figure><img src="image_314.png" /></figure>

368-U1502B-24R-3, 94-97 cm

<figure><img src="image_315.png" /></figure>

368-U1502B-24R-3, 94-97 cm

<figure><img src="image_316.png" /></figure>
<figure><img src="image_317.png" /><figcaption>Figure F30. Core composites of igneous lithologic Unit 2 basalts, highlighting all observed chilled contacts and (glassy) margins, as well as flow margins (ends of flow reflected by the presence of breccias or chert [for example] without a chilled contact), Hole U1502B. (Continued on next five pages.)</figcaption></figure>

dances of alteration-resistant elements such as Ti and Zr between these two units. Subunit 2a has TiO₂ contents of 2.2 ± 0.5 wt% and Zr concentrations of 161 ± 44 ppm, whereas Subunit 2b has TiO₂ contents of 1.8 ± 0.4 wt% and Zr concentrations of 131 ± 32 ppm. Although Subunit 2b has MgO contents more typical of basalts, other elements show that these basalts are still strongly affected by secondary alteration. CaO and Fe₂O₃ have wide ranges (CaO = 1–10 wt%; Fe₂O₃ = 3–14 wt%), and the highest values tend to be found in zones of significantly higher magnetic susceptibility (Figure F33) (see also Physical properties) that might indicate areas of less intense alteration.

Elevated S values (as high as 6.4 wt%) in many analyses are consistent with the widespread sulfide mineralization seen throughout igneous lithologic Unit 2. Visual inspection of the core shows that the sulfide mineralization is dominated by pyrite, but other sulfide minerals were also found: sphalerite ([Zn,Fe]S) in interval 368-U1502B-23R-3, 0-86 cm, chalcopyrite (CuFeS₂) in Sections 24R and 25R; and covellite (CuS) in interval 14R-2, 143–146 cm. The pXRF results can be used to determine the extent of Zn and Cu mobilization within the hydrothermal system at Site U1502. Cores 33R through 37R have high magnetic susceptibility and low Zn concentrations (average = 112 ± 22 ppm) that are typical of oceanic basalts (e.g., White and Klein, 2014). In the overlying cores (32R through 30R), there is a sudden jump to values as high as 800 ppm, and Zn remains at variably elevated levels until the upper part of Subunit 2a (Cores 4R–8R) (Figure F33), confirming significant pervasive mobilization of Zn. In contrast, there is little evidence for significant Cu

mobilization. Cu concentrations average ~80 ppm with values mostly below 300 ppm, and only one analysis gave high Cu (interval 17R-1, 57 cm = 2035 ppm).

Variations in ratios of immobile incompatible elements, such as Ti and Zr, can be used to assess whether there were any changes in parental magma compositions. The Ti/Zr ratio is the same in Subunit 2a (85 ± 14) and Subunit 2b (83 ± 13), implying that there were no significant changes in the melt generation process during the period of eruption of the basalts recovered at Site U1502. However, within Subunit 2b a jump in Ti/Zr ratios at 865 m (91 ± 12 above and 77 ± 10 below this depth) might indicate a small change in magma chemistry. Small-scale flow-by-flow differences in Zr and TiO₂ abundances are most likely due to variations in the extent of magmatic differentiation. The few measurements made on igneous lithologic Unit 1 show compositions quite distinct from those of igneous lithologic Unit 2, with a higher average Ti/Zr ratio of 218 ± 8 and Nb concentration of 19.4 ± 0.7 ppm (compared to 10.8 ± 2.6 ppm for Unit 2), as well as higher P₂O₅ and Cr but lower Zr and V abundances.

A high-resolution XRF core scanning data set for the Site U1502 volcanic rock section was produced onshore (see **Igneous and metamorphic petrology** in the Expedition 367/368 methods chapter [Sun et al., 2018a]) and can be retrieved as shipboard data from the IODP database (http://web.iodp.tamu.edu/LORE). At the time of this publication, assessment and discussion of this data set have not been concluded yet.
<figure><img src="image_318.png" /><figcaption>Figure F30 (continued). (Continued on next page.)</figcaption></figure>

## Interpretation

Igneous lithologic Unit 2 shows typical mid-ocean ridge basalt (MORB) flow morphologies with pillow basalts with quenched rims and sheet lavas, as well as a typical MORB mineralogy. The presence of agglutinated benthic foraminifers in the overlying Unit V and pillow basalts indicate that Unit 2 was emplaced in a marine environment. The agglutinated foraminifers indicate abyssal water depths (see **Biostratigraphy**). A combination of pillow basalts and sheet flows is common in submarine volcanic settings, with the change from sheet to pillow flows probably controlled by the magma discharge rate (Gregg and Fink, 1995; Fink, 2017) and the slope of the ridge axis (Kennish and Lutz, 1998). Some eruption sequences start with high flow rates and sheet morphologies and change to pillow flows as the rate declines (McClinton et al., 2014). For example, the 2005–2006 eruption on the East Pacific Rise (Soule et al., 2007) produced a flow that changed from inflated lobate and sheet morphologies in the flow interiors to pillow lavas at terminal flow fronts. This change was probably controlled by a reduction in the flow rate and a gentler slope away from the ridge axis (e.g., Kennish and Lutz, 1998). The chemistry of the freshest basalts (in areas with high magnetic susceptibility) also overlaps with typical MORB lava compositions (e.g., White and Klein, 2014). Compared to Site U1500 lavas (see **Igneous and metamorphic petrology** in the Site U1500 chapter [Stock et al., 2018]), igneous lithologic Unit 2 samples have slightly higher TiO₂ content (1.72 wt% versus 1.51 wt%), which may

indicate a slightly more enriched source or a more differentiated magma. Subunit 2b has multiple intervals where the plagioclase phenocryst content is more than 10% and in places as high as 50% (Figure F24). Basalts with >10% plagioclase phenocrysts are known as plagioclase ultraphyric basalts (PUBs) (Cullen et al., 1989) and are typically found at mid-ocean ridges with ultraslow to intermediate spreading rates and in off-axis seamounts (Flower, 1980; Lange et al., 2013), consistent with the spreading rates of 35–50 mm/y inferred for the SCS (Briais et al., 1993). Recent models (Lange et al., 2013) argue that PUBs form through magma interaction with preexisting lower crustal plagioclase-rich cumulates, and they are erupted only when ascent rates are sufficiently high (>0.5 cm/s) and where the conduit system lacks or bypasses an axial magma chamber.

The combination of intense alteration, multiple veins, and brecciation in the basalt sequence, particularly in Subunit 2a (e.g., interval 368-U1502B-4R-2, 84–93 cm), suggests alteration due to hydrothermal fluids. The abundant pyrite indicates that the fluids were rich in sulfur. Hydrothermal fluids can lead to hydraulic fracturing, which may have caused some of the brecciation in the unit (e.g., Jébrak, 1997). The alteration mineral assemblage belongs to the albite-epidote to lower greenschist facies and suggests high-temperature water-rock interaction. The presence of epidote and quartz shows that this interaction took place at temperatures higher than ~200°C (Reyes, 1990). This style of alteration is referred to as propylitic. The intensity of both alteration and brecciation de-
<figure><img src="image_319.png" /><figcaption>Figure F30 (continued). (Continued on next page.)</figcaption></figure>

creases with depth in the volcanic pile. Alteration in Subunit 2b is dominated by epidote, sulfides, and chlorite, whereas Subunit 2a shows more carbonate and clay alteration, which may indicate lower temperature alteration for the upper section of Unit 2.

The age of igneous lithologic Unit 2 can only be inferred from the biostratigraphy of the overlying sedimentary units because the sparse sedimentary material found between the basalts in the upper part of Unit 2 is barren of microfossils. Lithostratigraphic Unit IV (Core 368-U1502A-40R) is late Oligocene in age based on nanno-fossils and diatoms, and Unit V (Core 368-U1520B-3R) is inferred to be late Eocene or older in age based on agglutinated benthic foraminifers (see **Biostratigraphy**).

Igneous lithologic Unit 1 is only represented by a few basaltic fragments found between the early Miocene-late Oligocene litho-stratigraphic Unit II and the late Oligocene lithostratigraphic Unit IV and could represent a unit with a younger age than igneous lithologic Unit 2. It is possible that igneous lithologic Unit 1 might be

older than the late Oligocene if these fragments represent reworked material derived from nearby exposures of the underlying Unit 2 basalts. Nevertheless, there seem to be petrographic differences between igneous lithologic Units 1 and 2, although they are difficult to determine due to the high degree of alteration in both units. Unit 1 may have contained more olivine as phenocrysts, now completely altered, than Unit 2. In addition, preliminary shipboard analyses using ICP-AES and the pXRF indicate some distinct compositional differences between Units 1 and 2. Igneous lithologic Unit 1 has higher TiO₂, P₂O₅, Cr, and Nb contents and lower Zr and V contents than Unit 2. Another possible reworked source for the Unit 1 basalt fragments are late Oligocene-early Miocene basalts recovered from oil exploration wells (BY2 and BY7-1) in Baiyun Sag, about 200 km north-northwest of Site U1502 (Zhao et al., 2016). The relationship between igneous lithologic Units I and 2 and other basalts in the surrounding area will be evaluated further with postexpedition geochemical analysis.
<figure><img src="image_320.png" /><figcaption>Figure F30 (continued). (Continued on next page.)</figcaption></figure>
<figure><img src="image_321.png" /><figcaption>Figure F30 (continued). (Continued on next page.)</figcaption></figure>
<figure><img src="image_322.png" /><figcaption>Figure F30 (continued).</figcaption></figure>
<figure><img src="image_323.png" /><figcaption>Figure F32. XRD bulk mineralogy, Hole U1502B.</figcaption></figure>

<figure><img src="image_324.png" /></figure>
<caption>Table T3. Mineral percentages, Hole U1502B. View table in PDF format. Download table in CSV format.</caption>

Figure F33. pXRF compositional data for selected major element oxides and trace elements: Fe₂O₃, TiO₂/Zr, MgO, CaO, and Zn. Gray (in background) = magnetic susceptibility data measured on whole-round sections, vertical dashed lines = composition of average MORB, stars = thin section sample locations.

<figure><img src="image_325.png" /><figcaption>Table T4. Major element oxides and minor elements in rocks, Site U1502. View table in PDF format. Download table in CSV format.</figcaption></figure>

## Structural geology

Site U1502 is located on Ridge A, which represents a northeast-southwest-trending basement high within the COT zone, about 40 km east of Site U1499 and 15 km north of Site U1500 (Figure F1).

### Lithostratigraphic Unit I structures

Deformation structures are scarce in lithostratigraphic Unit I downhole to Core 368-U1502A-37R. This upper interval is weakly deformed to undeformed and characterized by horizontal to slightly dipping bedding (Figure F34). However, Sections 25R-3 through 25R-5 contain a convolutely bedded sequence (Figure F35), indicating soft-sediment deformation during mass movements (i.e., slump folding) before lithification of the sediments. Below Core 25R, a slight but consistent tilt with apparent dips on split core surfaces up to ~10° is recorded (Figure F34).

Starting from Core 37R at ~700 mbsf, a gradual increase in deformation structures was observed. Cores 37R, 38R, and 39R are characterized by microfaults evidenced by down-dip plunging striações and slickensides on exposed fracture surfaces at an average

frequency of about two per meter (Figure F36). These fractures are systematically associated with greenish halos from 3 to 40 mm wide within the well-consolidated brownish clay.

### Lithostratigraphic Unit II structures

No deformation structures were encountered in Unit II.

### Lithostratigraphic Unit III structures

Lithostratigraphic Unit III consists of an extremely silicified breccia that was poorly recovered with only a few decimeters in Sections 368-U1502B-2R-1 and 3R-1 and a single fall-in clast in the uppermost centimeters of Core 368-U1502A-40R (Lithostratigraphy). This breccia is essentially composed of quartz showing a fractal grain-size distribution evident in both macroscopic and microscopic observations, with a maximum clast size of ~2 cm and minimum clast size in the submillimeter scale (Figure F19). Quartz microstructures in the silicified matrix of this breccia show some domains governed by partially annealed fractal grain-size distribution. Other domains in the breccia may preserve evidence for initial sedimentary structures or microfossil shapes that were silicified, suggesting important fluid circulation. However, the nature of this breccia as either sedimentary, tectonic, or hydrothermal remains unconstrained.
<figure><img src="image_326.png" /><figcaption>Figure F34. Locations of apparent dip measurements of bedding, convolute bedding, fractures and microfaults, Hole U1502A.</figcaption></figure>

## Lithostratigraphic Unit IV structures

In Core 368-U1502A-40R, a small-scale transition from distributed to localized shear in unlithified sediments of Unit IV (biosiliceous-rich clay; see **Lithostratigraphy**) was encountered. Tilted bedding (~20°) with diffuse shear bands and a sigma clast in Section 40R-3 (Figures F34, F37B, F37C) indicate passive creep. From Section 40R-1 to Section 40R-5, an increasing occurrence of microfaults with striations and slickensides grades into a microfault network strongly affected by drilling disturbance (Figure F37D, F37E).

## Lithostratigraphic Unit V structures

No deformation structures were encountered in Unit V.

## Lithostratigraphic Unit VI structures

Unit VI is composed of basalts that were heterogeneously affected by brecciation and veining associated with the circulation of hydrothermal fluids. Subunit VIA is dominated by brecciated basalts/basaltic breccias, whereas Subunit VIB comprises a rather coherent series of lava flows crosscut by a multistage network of polymineralic veins.
<figure><img src="image_327.png" /><figcaption>Figure F35. A. Core composite image comprising two lithostratigraphic subunit boundaries (IB/IC and IC/ID), Hole U1502A. Subunit IC consists of a convolutely bedded sequence indicative for mass transport deposition. B. Detail of centimeter-scale slump folds.</figcaption></figure>

## Subunit VIA

Core 368-U1502A-41R, representing the very top of Unit VI, contains severely fractured and fragmented basalt (Figure F38). The fractures are locally filled by fine-grained unconsolidated greenish rocks. In Cores 368-U1502B-4R through 12R, Subunit VIA is characterized by diverse brecciated horizons associated with veins and microcracks (Figures F39, F40). Depending on the intensity of fragmentation and the clast/matrix ratio, the rocks are referred to as brecciated basalt or basaltic breccia (for more information about lithology definitions, see Igneous and metamorphic petrology in the Expedition 367/368 methods chapter [Sun et al., 2018a]). Such breccias contain angular clasts of variable size up to ~8 cm and are generally clast supported. A commonly observed feature is the jig-saw-puzzle structure, suggesting possible in situ fragmentation (Figure F39A, F39B).

Macro- and microscopic observations show that the breccia clasts consist mainly of sparsely to moderately plagioclase phyllitic basalts that were hydrothermally altered to variable degrees. How-

ever, some breccia varieties also contain sedimentary components (e.g., chert) as clasts and/or matrix constituents. In places, a possible initial veining episode predating the eventual brecciation is preserved in breccia clasts that are fragmented by silica, epidote, and carbonate veins (Figure F40A, F40B). The matrix between individual clasts is made of euhedral Ca-, Mg-, and/or Fe-carbonate crystals in a fine-grained groundmass of zeolite, clay minerals, and various Fe oxides (e.g., Section 368-U1502B-5R-CC; thin section [TS] 32; Figure F40C, F40D). The contact between the basalt clasts and the matrix is generally sharp with clearly defined edges. The matrix itself provides no evidence for a shear-induced brittle fabric (e.g., a Riedel-type fracture pattern). Locally, however, carbonate crystals are anhedral and exhibit undulose extinction at clast/matrix contacts, indicating a strained crystal lattice (Figure F40E, F40F). Thus, formation of such breccias likely involved fluid-assisted fracturing of basalt with limited displacement.

Section 368-U1502B-12R-1 displays a gradual transition from a clast-supported brecciated basalt to a matrix-supported basaltic
Figure F36. Microfaults in Subunit ID, Hole U1502A. A. Core composite image highlighting consistent steep orientation and high occurrence frequency (~2/m) of observed microfaults. B, C. Details of microfaults outlined in A showing striations/slickensides on exposed surfaces. Microfaults are consistently associated with greenish alteration zones of up to 4 cm width.

<figure><img src="image_328.png" /></figure>

breccia (Figure F39). This transition is accompanied by progressive rounding of clasts and severe grain-size reduction within the breccia matrix. Macroscopically, the structure of this rock therefore falls into our first-order definition of a fault gouge. However, thin section observations cannot provide support for a deformation process involving significant shear movements because the original matrix geometry is erased by static overgrowth of euhedral carbonate crystals, most likely as precipitates from a circulating fluid postdating the brecciation process (Figure F40G, F40H). Locally, the healed matrix is cut by a second generation of veins, indicating that multiple pulses of fluid-assisted fracturing were involved in the formation of this breccia.

## Subunit VIB

Subunit VIA is characterized by small-scale veins and micro-cracks only a few millimeters wide, showing a complex network

generally in direct relation with the brecciation. In contrast, Subunit VIB is characterized by a progressive decrease of breccia horizons and the development of different vein types, almost always poly-mineralic. Notably, the composition and orientation of the veins change throughout Subunit VIB (Figure F41). Although numerous small-scale veins often related with major ones can be observed, only the main veins are represented in this figure, therefore providing only a first-order trend. In Cores 368-U1502B-12R through 37R, three vein types were identified: (1) composite silica-rich vein, (2) composite epidote-rich vein, and (3) composite carbonate and/or silica-rich vein. For more information about their mineralogy, see **Igneous and metamorphic petrology**.

The first category includes silica-rich veins that are not particularly abundant. They are characterized by irregular shapes and single to branched connectivity (Figures F42, F43) and often appear
Figure F37. Deformation structures in Unit IV, Hole U1502A. A. Core composite image with locations of detailed images in B–E. B, C. Distributed shear geometries with shear bands, sigma clast, and tilted bedding. D. Striations/slickensides on exposed microfault surface. E. Dense network of shear bands and fractures enhanced by drilling disturbance.

<figure><img src="image_329.png" /></figure>

crosscut by composite epidote-rich veins (Figure F42A), whereas locally the opposite relations are also inferred. These epidote-rich veins, observed from Core 10R downhole, are associated with sulfides, silica, chlorite, various clay minerals, and zeolites. These veins are generally vertical to subvertical, from 1 to 25 mm wide, and typically irregular in shape. Individual veins are continuous for only short distances (less than a few tens of centimeters) and commonly terminate on other veins, forming an interconnected network (Figure F43D), although branched geometries are also common (Figure F43B, F43C, F43F). Locally, the epidote vein displays a radial geometry in relation to a pillow margin (Figure F43E). From Core 35R downhole, this vein type becomes gradually less abundant. Eventually, these composite epidote-rich veins are overprinted by a third type composed of composite carbonates and/or silica-rich veins (Figure F42B, F42C). These veins show various sizes ranging from 1 to 10 mm with various orientations. In Cores 35R–37R, these veins are more abundant and are associated with shallow to moderate dips (<50°) with a straight shape (Figure F43). They record a texture from massive to slip fiber showing single to branched to network connectivity.

## Interpretation

The upper part of the sedimentary record encountered at Site U1502 is essentially devoid of deformation structures apart from Core 368-U1502A-25R, which contains a convolutely bedded sequence in Sections 25R-3 through 25R-5 (Subunit IC). The formation of this soft-sediment deformation structure most probably involved mass movements related to gravitational instabilities on a slope in close vicinity of a basement high (i.e., Ridge A).

A gradual downhole increase of predominantly brittle deformation is recorded within the sediments directly overlying the volcanic basement (Cores 368-U1502A-37R through 40R), whereas severe brecciation was observed within the uppermost ~50 m of the basement top (Cores 368-U1502B-4R through 12R). The observed deformation structures are interpreted to be related to the emplacement history of the basalt and the subsequent development of a hydrothermal system. Assuming such a setting, local strain accumulations resulting from possible thermal reequilibration and triggered by the circulation of fluids in both magmatic rocks and sedimentary cover may well be expected.
<figure><img src="image_330.png" /><figcaption>Figure F38. Strongly fractured and fragmented basalt filled by fine-grained unconsolidated greenish rock, top of Unit VI.</figcaption></figure>

The numerous brecciated horizons observed in Subunit VIA are most probably related to fluid-assisted fracturing associated with hydraulic (and) in situ fragmentation of basalts. In that context and from a mechanical perspective, rock failure may have occurred in the tensile regime associated with low differential stress and high fluid pressure. Such rocks are commonly described in hydrothermal systems (e.g., Jébrak, 1997; Clark and James, 2003) and are identified as hydrothermal breccias. In some cases, however, a clear distinction between hydrothermal and tectonic breccias remains unconstrained, as it does in Section 368-U1502B-12R-1, which marks the boundary between Subunits VIA and VIB. Indeed, macroscopic observations from this section are compatible with a fault breccia, showing a gradual increase of the brittle deformation passing from a clast-supported cataclasite to a matrix-supported fault gouge. However, the identification of characteristic microstructures resulting from shear deformation are precluded by a pervasive fluid circulation associated with replacement, destabilization, and recrystallization of the initial texture. Therefore, at its present stage, two possible scenarios are considered regarding the formation mechanism of this breccia: (1) the breccia results essentially from hydrothermal fluid-assisted brecciation associated with the replacement and destabilization of the initial basalts, or (2) the breccia represents a fault zone with cataclastic deformation processes and was subsequently overprinted by pervasive hydrothermal fluid circulation.
Figure F39. Breccias in Unit VI, Hole U1502B. A, B. Brecciated basalt with jigsaw-puzzle structure. C. Core composite image showing spatial relationships between brecciated basalts and basaltic breccias. D. Basaltic breccia with fractal grain size distribution and high clast to matrix ratio. E. Initial stage of breccia matrix development, note how amount of angular clasts in each crack increases with crack width. F. Gradual transition from clast-supported brecciated basalt to matrix-supported basaltic breccia, macroscopically resembling a transition from cataclastic fabric to fault gouge.

<figure><img src="image_331.png" /></figure>
Figure F40. Progressing stages of brecciation process in macro- and microscopic view. A, B. Initial carbonate veining in basalt. C, D. Altered basalt clasts forming a jigsaw-puzzle texture within healed matrix. E, F. Altered basalt clasts surrounded by matrix carbonates with undulose extinction (i.e., strained matrix). G, H. Statically overgrown matrix by euhedral carbonate crystals, whereas altered basalt clast is crosscut by vein composed of fine-grained minerals consisting mainly of carbonate, zeolite, clay minerals and various Fe oxides.

368-U1502B-4R-2, 15-21 cm, TS 31

<figure><img src="image_332.png" /></figure>

368-U1502B-5R-CC, 26-33 cm, TS 32

<figure><img src="image_333.png" /></figure>

368-U1502B-6R-1, 1-6 cm, TS 34

<figure><img src="image_334.png" /></figure>

368-U1502B-12R-1, 83-86 cm, TS 39

<figure><img src="image_335.png" /></figure>

Figure F41. Calculated true dip angles of composite veins crosscutting brecciated basalts, basaltic breccias, and lava flows showing broad overall trends, as only principal veins were measured, Unit VI. For example, Subunit VIA contains numerous small-scale (millimeter) veins and microcracks associated with brecciation that generally show highly variable orientations. Furthermore, their mineralogy usually consists of fine-grained minerals that cannot be unambiguously identified, so only major veins are represented. Note the predominantly steep orientations of epidote-rich veins, and in contrast, the rather shallowly dipping silica- and carbonate-rich veins in lower-most part of Hole U1502B starting from Core 30R. CRF = core reference frame.

U1502B hole summary

<figure><img src="image_336.png" /></figure>
Figure F42. Principal generations of veins observed in Subunit VIB according to crosscutting relationships. A. Composite silica-rich vein crosscut by composite epidote-rich vein. B. Composite carbonate and/or silica-rich vein crosscutting branched composite epidote-rich veins with pyrite. C. Vertical composite epidote-rich vein crosscut by subhorizontal straight composite carbonate vein.

<figure><img src="image_337.png" /></figure>

Figure F43. Distinct vein shapes, connectivity, and mineral compositions observed, Subunit VIB. A. Straight, single carbonate vein (white) and silica vein (gray). B. Branched thick composite epidote-rich vein on whole round. C. Composite of epidote and pyrite-rich veins in whole round. D. Network of composite epidote-rich veins. E. Composite epidote-rich veins showing radial geometry near chilled margin. F. Branched subvertical composite veins (epidote, sulfide, silica, and carbonate).

<figure><img src="image_338.png" /></figure>

## Biostratigraphy

All core catcher samples from Hole U1502A and selected samples from Hole U1502B were analyzed for calcareous nannofossils, foraminifers, diatoms, and ostracods. Additional samples were taken from intervals within the working-half cores when necessary to refine the ages. The preservation of calcareous microfossils is mostly good to moderate in the upper sections (Cores 368-U1502A-2R through 13R) and moderate to poor in the lower sections (Cores 14R through 41R). Planktonic foraminifers and calcareous nannofossils are abundant to common in Cores 2R through 10R, 25R, and 37R through 39R and rare to barren in all the other sections. Diatoms were only observed in Sections 40R-1 through 40R-3. In Hole U1502B, foraminifers were observed only in Section 3R-1 and are represented by well-preserved agglutinated deepwater taxa. Ostracods are extremely rare and were observed only in Samples 368-U1502A-3R-CC to 5R-CC.

Fourteen biostratigraphic events from the planktonic foraminifers and calcareous nannofossils were used to provide an age-depth model for Site U1502 from the late Miocene to late Oligocene (Tables T5, T6; Figure F44). The Pliocene/Miocene boundary is placed tentatively within Core 368-U1502A-2R, but it may be above the cored section, and the Miocene/Oligocene boundary was determined to lie between Samples 39R-CC and 40R-1, 7 cm. Sedimentation rates varied from ~39 mm/ky in the late Miocene to ~10 mm/ky in the middle and early Miocene.

No planktonic microfossils were found in Hole U1502B, but Sample 3R-1, 94–99 cm, from a sandy interval) revealed abundant and diverse abyssal agglutinated benthic foraminifers. The taxonomic composition of this agglutinated benthic foraminifer assemblage suggests a late Eocene age for this interval.

## Calcareous nannofossils

The calcareous nannofossil biostratigraphy for Hole U1502A is based mainly on the analysis of core catcher samples, with occasional toothpick samples from the working-half cores to better constrain the biostratigraphic events (Table T7). Calcareous nannofossils are abundant and well preserved in Cores 2R through 6R, deteriorating to moderate preservation with abundant to common occurrences in Cores 7R through 12R. Calcareous nannofossils are absent in Cores 13R through 36R with the exception of a few samples where they are present but the assemblages are affected by strong dissolution. In contrast, nannofossils are abundant in Cores 37R through 40R. Reworking, overgrowth, and fragmentation were observed in most samples from Hole U1502A, whereas heavy dissolution occurred in discoasterid-dominated assemblages, if present, in the middle part of the sequence. Nine nannofossil biostratigraphic datums were recognized from the late Oligocene through late Miocene sediment sequence in Hole U1502A (Table T5). The oldest nannofossil event identified in Hole U1502A is the last appearance datum (LAD) of *Sphenolithus predistentus* (26.93 Ma) observed in Sample 40R-1, 69 cm (739.79 m), whereas the first appearance datum (FAD) of *Sphenolithus distentus* (30.0 Ma) cannot be determined due to the nannofossil-barren interval from the immediately underlying sample to the bottom of Hole U1502A, indicating an age older than 26.93 Ma but possibly younger than 30.0 Ma. All core catchers and toothpick samples from within the work-
<table><caption>Table T5. Calcareous nannofossil biostratigraphic events, Site U1502. O = occurrence, T = top/last appearance datum, B = bottom/first appearance datum.
Download table in CSV format.</caption><thead><tr><th rowspan="2"></th><th rowspan="2">Epoch</th><th rowspan="2">Biozone (Martini, 1971)</th><th rowspan="2">Calcareous nannofossil event</th><th colspan="2">Top</th><th colspan="2">Bottom</th><th colspan="2">Age model</th></tr><tr><th>Core, section, interval (cm)</th><th>Depth CSF-A (m)</th><th>Core, section, interval (cm)</th><th>Depth CSF-A (m)</th><th>Depth (m)</th><th>Age (Ma)</th></tr></thead><tbody><tr><td rowspan="7">Miocene</td><td rowspan="4">late</td><td rowspan="4">NN11</td><td>O Discoaster quinqueramus</td><td>368-U1502A-</td><td></td><td>368-U1502A-</td><td></td><td></td><td></td></tr><tr><td>T Nicklithus amplificus</td><td>4R-CC</td><td>394.67</td><td>2R-CC</td><td>376.81</td><td>376.81</td><td>&gt;5.59</td></tr><tr><td>B Amaurolithus spp.</td><td>9R-CC</td><td>450.22</td><td>5R-CC</td><td>403.85</td><td>408.95</td><td>5.94</td></tr><tr><td>B Discoaster berggrenii</td><td>13R-2, 89-90</td><td>483.01</td><td>10R-CC</td><td>458.81</td><td>454.52</td><td>7.42</td></tr><tr><td rowspan="2">middle</td><td rowspan="2">NN6</td><td>T Cyclicargolithus floridanus</td><td>29R-CC</td><td>643.46</td><td>30R-CC</td><td>653.27</td><td>648.37</td><td>11.85</td></tr><tr><td>T Triquetrorhabdulus carinatus</td><td>36R-CC</td><td>710.46</td><td>37R-CC</td><td>719.62</td><td>715.04</td><td>18.28</td></tr><tr><td rowspan="3">early</td><td rowspan="3">NN2</td><td>B Sphenolithus belemnos</td><td>38R-CC</td><td>728.32</td><td>39R-CC</td><td>734.87</td><td>731.60</td><td>19.03</td></tr><tr><td>B Sphenolithus disbelemnos</td><td>39R-CC</td><td>734.87</td><td>40R-1, 7</td><td>739.17</td><td>737.02</td><td>22.76</td></tr><tr><td>T Sphenolithus predistentus</td><td>39R-CC</td><td>734.87</td><td>40R-1, 7</td><td>739.17</td><td>737.02</td><td>26.93</td></tr><tr><td colspan="10"></td></tr><tr><td colspan="10"></td></tr><tr><td colspan="10"></td></tr></tbody></table>

<table><caption>Table T6. Planktonic foraminifer biostratigraphic events, Site U1502. O = occurrence, T = top/last appearance datum, B = bottom/first appearance datum.
Download table in CSV format.</caption><thead><tr><th rowspan="2">Epoch</th><th rowspan="2">Biozone (Blow, 1969)</th><th rowspan="2">Planktonic foraminifer event</th><th colspan="2">Top</th><th colspan="2">Bottom</th><th colspan="2">Age model</th></tr><tr><th>Core, section</th><th>Depth CSF-A (m)</th><th>Core, section</th><th>Depth CSF-A (m)</th><th>Depth (m)</th><th>Age (Ma)</th></tr></thead><tbody><tr><td rowspan="5">Miocene</td><td>early Pliocene</td><td>N19</td><td>O Globoturborotalita nepenthes</td><td>368-U1502A-</td><td></td><td>368-U1502A-</td><td></td><td></td><td></td></tr><tr><td></td><td>N17</td><td>B Globorotalia margaritae</td><td>8R-CC</td><td>432.74</td><td>2R-CC</td><td>376.81</td><td>376.81</td><td>&gt;4.37</td></tr><tr><td></td><td>N8</td><td>B Globoturborotalita nepenthes</td><td>30R-CC</td><td>653.27</td><td>9R-CC</td><td>450.22</td><td>441.48</td><td>6.08</td></tr><tr><td></td><td>N6</td><td>T Catapsydrax dissimilis</td><td>36R-CC</td><td>710.46</td><td>31R-CC</td><td>662.75</td><td>658.01</td><td>11.63</td></tr><tr><td></td><td>N4</td><td>T Paragloborotalia kugleri</td><td>37R-CC</td><td>719.62</td><td>37R-CC</td><td>719.62</td><td>715.04</td><td>17.54</td></tr><tr><td colspan="10"></td></tr><tr><td colspan="10"></td></tr></tbody></table>

ing-half cores of Hole U1502B are barren of calcareous nannofossils.

## Miocene

Four events occur in the late Miocene strata of Hole U1502A. The top of Zone NN11 is defined by the LAD of *Discoaster quinqueramus* (5.59 Ma), which was observed in the uppermost Sample 2R-CC (376.81 m), indicating an age older than 5.59 Ma. The LAD of *Nicklithus amplificus* (5.94 Ma) was found in Sample 5R-CC (403.85 m). The FAD of *Amaurolithus* spp. (7.42 Ma) was found in Sample 10R-CC (458.81 m). The bottom of Zone NN11 is placed in Sample 13R-2, 89-90 cm (483.01 m), where the apparent FAD of *Discoaster berggrenii* (8.29 Ma) was found right above a nannofossil-barren interval. The LAD of *Cyclicargolithus floridanus* (11.85 Ma) within Zone NN6 is placed in Sample 30R-CC (653.27 m), and this sole event in the middle Miocene strata of Hole U1502A approximates the top of Zone NN6 (11.90 Ma) and the late/middle Miocene boundary. Three bioevents were found in the early Miocene strata of Hole U1502A. The top of Zone NN2 is marked by the LAD of *Triquetrorhabdulus carinatus* (18.28 Ma), which was observed in Sample 37R-CC (719.62 m) immediately below a nannofossil-barren interval; the FAD of *Sphenolithus belemnos* (19.03 Ma; Zone NN2) was also found in this same sample. The base of Zone NN2 and the Miocene/Oligocene boundary are inferred by the FAD of *Sphenolithus disbelemnos* (22.76 Ma), which was observed in Sample 39R-CC (734.87 m). Notably, the co-occurrence of *Sphenolithus delphix* (23.11–23.21 Ma) in this same sample and the LAD of *Reticulofenestra bisecta* >10 µm (23.13 Ma) in the immediately underlying Sample 40R-1, 7 cm (739.17 m), may be the result of the sample size (5 cm thick) and/or the presence of a hiatus.

## Oligocene

One Oligocene bioevent was identified in Hole U1502A. The LAD of *S. predistentus* (26.93 Ma) within Zone NP24 was observed in Sample 40R-1, 7 cm (739.17 m). The occurrence of *S. distentus* (first occurrence dated to 30.0 Ma) immediately overlies a barren interval, indicating a possible deeper position of its FAD and therefore an age younger than 30 Ma at 739.79 m (Sample 40R-1, 7 cm).

## Planktonic foraminifers

All core catcher sediment samples from Holes U1502A and U1502B were processed for foraminiferal analyses. Additional samples were taken and analyzed from the split cores to supplement the biostratigraphic work (Table T8). Planktonic foraminifers are moderately to well preserved and abundant to common in samples from Cores 368-U1502A-2R through 10R and moderately to poorly preserved or even recrystallized and less abundant in Cores 11R through 13R, 25R, and 29R through 31R. Samples from Cores 14R through 24R, 26R through 28R, 32R through 36R, and 40R through 41R and all samples from Hole U1502B are barren of planktonic foraminifers, whereas planktonic foraminifers in samples from Cores 368-U1502A-37R through 39R are poorly preserved with relatively high abundance.

Five planktonic foraminiferal datums spanning from the late Pliocene to the early Miocene were recognized in Hole U1502A (Table T6). The planktonic foraminiferal biostratigraphy for Hole U1502A was established by these events and the planktonic foraminiferal assemblages at successive depth intervals. No planktonic foraminiferal biostratigraphy events are recognized for Hole U1502B.

The middle–late Miocene planktonic foraminiferal assemblage in samples from Cores 368-U1502A-3R through 29R is dominated
<figure><img src="image_339.png" /><figcaption>Table T7. Distribution of calcareous nannofossils, Site U1502. **View table in PDF format. Download table in CSV format.**</figcaption></figure>

<caption>Table T8. Distribution of planktonic foraminifers, Site U1502. **View table in PDF format. Download table in CSV format.**</caption>

by *Sphaeroidinellopsis seminulina*, *Globoturborotalita nepenthes*, and *Sphaeroidinellopsis kochi* (Plate P1), whereas *Dentoglobigerina altispira* is also a common species in Samples 3R to 9R. The early

Miocene–Oligocene planktonic foraminiferal assemblage in Samples 37R to 40R is dominated by *Catapsydrax dissimilis* and *Globoquadrina venezuelana*.

The presence of *S. seminulina* and *G. nepenthes* in Sample 2R-CC (376.81 m) indicates the top age of this hole should be older than 4.37 Ma. Four other events occur in the Miocene strata of Hole U1502A. The FAD of *Globorotalia margaritae* (6.08 Ma) was found in Sample 8R-CC (432.74 m), the FAD of *G. nepenthes* (11.63 Ma) was found in Sample 30R-CC (653.27 m), the LAD of *C. dissimilis*
Plate P1. Principal planktonic foraminifers (Hole U1502A), benthic foraminifers (Hole U1502B), and ostracods (Hole U1502A) used in biostratigraphy and paleoecology analyses, Site U1502. 1–10, 18: scanning electron microscope (SEM) (scale bar = 100 µm); 11–17: optical (scale bar = 1 mm). 1. *Sphaeroidinellopsis seminulina* (U1502A-5R-CC). 2, 3. *Globorotalia margaritae* (4R-CC). 4. *Globoturborotalita nepenthes* (5R-CC). 5. *Sphaeroidinellopsis kochi* (3R-CC). 6. *Globoquadrina venezuelana* (39R-CC). 7. *Catapsydrax dissimilis* (38R-CC). 8. *Paragloborotalia kugleri* (39R-CC). 9–17. U1502B-3R-1, 94–99 cm; (9) *Psammosiphonella cylindrica*; (10) *Cribrostomoides* sp.; (11) *Ammodiscus* cf. *Ammodiscus cretaceus* sp. with *Ammolagena clavata* attached; (12) *Ammobaculites* sp. aff. *Ammobaculites agglutinans*; (13, 14) *Reticulophragmium amplectens*; (15, 16) *Reticulophragmium gerochi*; (17) *Psamminopelta gradsteini*. 18. *Rugocythereis horrida* (U1502A-3R-CC).

<figure><img src="image_340.png" /></figure>

(17.54 Ma) was found in Sample 37R-CC (719.62 m), and the LAD of *Paragloborotalia kugleri* (21.12 Ma) within Zone N4 occurs in Sample 38R-CC (728.32 m).

## Benthic foraminifers

In Hole U1502B, only one studied sample (interval 3R-1, 94–99 cm) yielded agglutinated benthic foraminifers. In this sample, the agglutinated foraminifers are abundant and the assemblage is dominated by typical abyssal species (e.g., *Psammosiphonella cylindrica*, *Cribrostomoides* sp., *Ammodiscus* cf. *Ammodiscus cretaceous*, *Am-

molagena clavata*, *Ammobaculites* sp., *Reticulophragmium amplectens*, *Reticulophragmium gerochi*, *Psamminopelta gradsteini*, and *Rugocythereis horrida*). Compared to the calcareous microfossil assemblage found in Hole U1502A, the agglutinated assemblage indicates that either a very deepwater paleodepth or a shallower CCD existed during this time. The taxonomic composition of the benthic assemblage, in particular the association between *A. clavata*, *Ammodiscus* sp., *P. gradsteini*, *R. amplectens*, and *R. gerochi*, indicates an age of late Eocene (M. Kaminski, pers. comm., 2017) for this layer. Moreover, the on-growth of *A. clavata* on *Ammodiscus* sp.
<caption>Table T9. Distribution of diatoms, Site U1502. View table in PDF format. Download table in CSV format.</caption>

<table><caption>Table T10. Diatom biostratigraphic event, Site U1502. O = occurrence. Download table in CSV format.</caption><thead><tr><th rowspan="2">Epoch</th><th rowspan="2">Diatom zonation<br>(Barron, 1985)</th><th rowspan="2">Diatom event</th><th colspan="2">Top</th><th colspan="2">Bottom</th><th colspan="2">Age model</th></tr><tr><th>Core, section,<br>interval (cm)</th><th>Depth CSF-A<br>(m)</th><th>Core, section,<br>interval (cm)</th><th>Depth CSF-A<br>(m)</th><th>Depth<br>(m)</th><th>Age range<br>(Ma)</th></tr></thead><tbody><tr><td>Oligocene</td><td><i>Rocella vigilans</i>-<i>Rocella gelida</i></td><td>O <i>Rocella vigilans</i></td><td>40R-1, 32</td><td>739.42</td><td>40R-2, 136</td><td>741.93</td><td>741.93</td><td>23.9-29.6</td></tr></tbody></table>

(Plate P1, figure 11) was previously reported from Eocene coarse pyroclastic sediments overlying basement at Ocean Drilling Program (ODP) Site 643 (plate 21.3 in Kaminski and Gradstein, 2005).

## Ostracods

The deepwater species *R. horrida* was found as single valves, one each in Samples 368-U1502A-3R-CC, 4R-CC, and 5R-CC.

## Diatoms

The core catchers of Hole U1502A were processed into smear and strewn slides for diatom analysis with 57 additional toothpick samples taken from selected intervals of the working-half cores for further resolution. All core catcher samples and most additional samples were found to be barren of diatoms (Table T9). Toothpick samples from the top of Section 40R-1 contain abundant but poorly preserved diatoms, decreasing in abundance to barren in Section 40R-4. *Chaetoceros* spp. are few to common in these sections, with few *Cyclotella* spp., *Distephanosira* *architecturalis*, *Paralia sulcata*, and *Thalassiothrix* spp. Sections 40R-1 through 40R-3 have an early Oligocene diatom assemblage; the presence of *Rocella vigilans* indicates the sections are between 30 to 24 Ma in age (Table T10).

## Paleomagnetism

### Shipboard measurements

Shipboard paleomagnetic investigations combined three complementary approaches (see **Paleomagnetism** in the Expedition 367/368 methods chapter [Sun et al., 2018a]): (1) measurement and in-line AF demagnetization of archive-half sections on the pass-through 2G Enterprises SRM at 2.5 and 2 cm spacing for sediments and basalts, respectively; (2) measurement and thermal demagnetization of oriented discrete samples on the SRM; and (3) measurement and AF demagnetization of oriented discrete samples on the spinner magnetometer (AGICO JR-6A). For sedimentary units, 40 representative discrete samples were collected for thermal demagnetization and 15 for AF demagnetization from sections where recovery was higher than ≈20%. For igneous basement units, 8 discrete samples were collected for thermal demagnetization and 41 for AF demagnetization.

First, we used scalar data from discrete samples that were thermally demagnetized up to 700°C or AF demagnetized up to 120 mT to assess the nature of the magnetic assemblage. Second, we used the directional and intensity data of archive-half sections measured after in-line AF demagnetization of 25 mT to determine magnetic polarity along the core. The thermal instability of the sedimentary rocks combined with the lack of laboratory shielding prevented the use of thermal demagnetization to confirm magnetic polarities. Thermal demagnetization of samples from the igneous basement was successful due to intrinsically higher NRM in basaltic rocks. Directional data were analyzed using Zijderveld diagrams (Zijderveld, 1967), and the characteristic remanent magnetization (ChRM) direction(s) were calculated by principal component analysis (PCA) (Kirschvink, 1980) using PuffinPlot (version 1.03, 23 April 2015) (Lurcock and Wilson, 2012).

### Demagnetization behavior of sedimentary rocks and implications for magnetic assemblages

Units: IA to V

Depths: Hole U1502A= 375.00-750.67 m core depth below sea-floor, Method A (CSF-A); Hole U1502B = 727.79-739.16 m CSF-A.

Cores 368-U1502A-2R through 41R; 368-U1502B-2R through 3R

The data from thermal demagnetization at the 95°, 120°, and 200°C heating steps were irreparably lost because of an error in drift-and-background corrections on the SRM. Subsequent measurements at higher temperature steps were successfully performed on 30 representative samples using the JR-6A spinner magnetometer. However, stepwise thermal demagnetization of the NRM above 200°C failed because instead of losing magnetization the samples gained magnetization (Figure F45A). However, these failed experiments show that the behavior of the sedimentary material is strongly associated with its color (red or green), which primarily reflects distinct mineral assemblages.

Red beds (e.g., discrete Sample 368-U1502A-37R-5, 122-124 cm) initially display normal behavior marked by a decrease in magnetization up to 350°C, followed by an anomalous increase in magnetization from 350° to 600°C and finally a normal decrease above 600°C (Figure F45). The increase in magnetization results from at least one thermally induced reaction that forms, upon heating, one (or several) magnetically remanent phases. Although such reactions are common in sedimentary materials, the acquisition of magnetization in the magnetically shielded TD-48 SC oven is anomalous. The gain in magnetization does not increase with the duration of time before measurement (outside the oven), which suggests that the samples are remagnetized at high temperature in the ≈60 nT residual field (measured with Applied Physics System APS 550 fluxgate probe at room temperature) present in the oven. The magnetic phase formed by the heating-induced reaction must be able to acquire a high chemical remanent magnetization (CRM) to contribute so significantly to bulk magnetization. Because magnetization generally decreases above 600°C, this phase is likely to consist primarily of magnetite (Fe₃O₄; saturation magnetization: σ ≈ 92 Am²/kg) and hematite (αFe₂O₃; σ ≈ 0.4 Am²/kg). Potential precursors to thermally induced magnetite and hematite in sedimentary rocks include siderite (paramagnetic), greigite (Fe₃S₄; σ ≈ 25 Am²/kg), and pyrite (paramagnetic). Although siderite (FeCO₃) has been reported in red bed interval 368-U1502A-33R-2, 8-15 cm, and detected by XRD in Sample 33R-3, 25-27 cm (see Lithostratigraphy), it does not constitute a major or ubiquitous phase in the sedimentary units of Holes U1502A or U1502B, and therefore its breakdown into magnetization(s) were calculated by principal component analysis (PCA) (Kirschvink, 1980) using PuffinPlot (version 1.03, 23 April 2015) (Lurcock and Wilson, 2012).
Figure F45. Thermal demagnetization showing relative magnetization with respect to NRM, Hole U1502A. A. Behavior of sedimentary material strongly reflects distinct mineral assemblages. B. Greenish beds with decrease of magnetization up to 350°C followed by increase in magnetization. C. Reddish bed with anomalous increase in magnetization from 350°C. D. Reddish beds with anomalous increase in magnetization from 350° to 600°C.

<figure><img src="image_341.png" /></figure>

tite around 350°C (Ellwood et al., 1986; Hirt and Gehring, 1991; Pan et al., 2000) is unlikely to cause the observed large gain in magnetization. The breakdown of greigite into magnetite around 330°C (Roberts et al., 2011) would cause an insufficient increase in magnetization (≈ 3.68). Finally, the breakdown of pyrite (FeS₂), a common paramagnetic phase in sedimentary units of Holes U1502A or U1502B, into magnetite and hematite (αFe₂O₃) around 350°C (Telling and Hrouda, 1993) best accounts for the substantial increase in magnetization between 350° and 600°C:

$$3 \text{FeS}_2 + 8 \text{O}_2 = \text{Fe}_3\text{O}_4 + 6 \text{SO}_2$$

for pyrite + oxygen = magnetite + sulfur dioxide, and

$$2 \text{FeS}_2 + 5.5 \text{O}_2 = \alpha \text{Fe}_2\text{O}_3 + 4 \text{SO}_2$$

for pyrite + oxygen = hematite + sulfur dioxide.

The green beds (e.g., discrete Sample 368-U1502A-14R-3, 21–23 cm) display significantly higher reactivity to heating (Figure F45A). These samples consistently display a very large magnetization enhancement, up to 600 times the NRM, above 200°C.

Stepwise AF demagnetization was performed up to 120 mT. Most samples display a demagnetization behavior corresponding to the color of sedimentary material (red or green), although some sediments of intermediate color show mixed behaviors. Red sediments show an initial 50% loss of magnetization by 25 mT (Figure F46), a soft behavior indicative of magnetite or titanomagnetite in the multidomain to pseudosingle-domain size range. This behavior changes drastically above 30 mT and up to 120 mT. The reddish

color of the sediment combined with this magnetically harder behavior supports the presence of hematite in these sediments. In contrast, green sediments exhibit a very soft AF behavior characterized by rapid demagnetization below 15 mT, which is typical of multidomain–pseudosingle domain magnetite or titanomagnetite. Many samples gain magnetization above 70 mT, a phenomenon attributed to gyroremanent magnetization (GRM) or rotational remanent magnetization (e.g., Thompson, 1990; Snowball, 1991; Hu et al., 2002) that is common in pyrrhotite and greigite. The low saturation isothermal remanent magnetization to magnetic susceptibility (χ) ratios of these green sediments (Table T11) does not support the presence of greigite. Generally, AF treatments on discrete samples removed the steep overprint component more successfully with respect to the SRM in-line treatment on the archive sections (Figures F47, F48, F49).

## Demagnetization behavior, rock magnetism data, and implications for magnetic assemblages of igneous rocks

Unit: igneous lithologic Unit 2

Depth: 739.16–920.95 m CSF-A

Cores 368-U1502B-4R through 37R

The igneous rocks of Hole U1502B consist of various types of altered basalt (see **Igneous and metamorphic petrology**). The basalt cores have an average initial intensity (NRM) of magnetization one order of magnitude higher than sediments (Figure F48). This leads to successful AF and thermal demagnetization for both archive halves and discrete samples. Typical examples of demagnetized
Figure F46. AF demagnetization of red and green samples, Hole U1502A. Reddish and greenish sediments are represented with red and green symbols, respectively. Reddish sediments show magnetically harder behavior that supports the presence of hematite (9R-1, 133–135 cm; 39R-1, 65–67 cm). Green sediments show very soft behavior typical of magnetite or titanomagnetite. Some sediments of intermediate color show mixed behaviors (25R-3, 135–137 cm; 36R-5, 41–43 cm). The gain in magnetization above 70 mT is attributed to GRM common in pyrrhotite and greigite (12R-5, 139–141 cm; 13R-1, 44–46 cm; 14R-3, 18–20 cm).

<figure><img src="image_342.png" /></figure>

zation behavior and polarity interpretation are shown in Figure F50. AF and thermal demagnetization of twin samples collected at the same depth interval lead to comparable results. AF demagnetization shows two types of behaviors, with 90% magnetization loss either before 45 mT (soft) or 100 mT (hard).

Magnetic susceptibilities obtained by point measurement on section halves range widely from $κ ≈ 18 × 10⁻⁶ SI$ to 33,000 × 10⁻⁶ SI, with median values around $327 × 10⁻⁶ SI$ (Figure F51). The distribution of magnetic susceptibility from 728 to 920.8 m CSF-A shows a bimodal distribution with two asymmetric maxima, negatively skewed toward lower $κ$ values (histogram in Figure F51). Progressive increasing alteration of magmatic magnetite into maghemite and hematite, both phases with lower intrinsic magnetic susceptibilities, best explains the skewness of the $κ$ distribution.

The bimodal nature of the $κ$ distribution and the variations of $κ$ with depth have a different origin. Although magnetic susceptibility $κ$ decreases with increasing alteration, such a process should produce a continuum of $κ$ values from fresh to altered rocks instead of a bimodal distribution. The $Fe_2O_3$ versus $κ$ diagram (Figure F52) indeed shows a gap in magnetic susceptibility, which further questions a chemical alteration origin for the $κ$ bimodality. Several 4 to 13 m thick discrete intervals of consistently high magnetic susceptibility occur throughout these volcanic rocks (gray intervals in Figure F51). These intervals show abrupt variations of $κ$ with depth. These high-$κ$ intervals could be interpreted as gravitational accumulation of magnetite toward flow bottoms, as has been observed in basalt flows elsewhere (e.g., Caballero-Miranda et al., 2016). Alternatively, since basaltic flows often develop a tabular structure with more massive lower parts (e.g., Boiron et al., 2013), the lack of

<table><caption>Table T11. Rock magnetic properties of representative samples, Hole U1502A. Download table in CSV format.</caption><thead><tr><th>Core, section, interval (cm)</th><th>SIRM<sub>23T</sub> (A/m)</th><th>χ (10<sup>-6</sup>), at 300 A/m</th><th>SIRM/χ (kA/m)</th></tr></thead><tbody><tr><td colspan="4"><strong>368-U1502A-Red sediment</strong></td></tr><tr><td>39R-1, 65–67</td><td>1.287</td><td>176</td><td>7.313</td></tr><tr><td>9R-1, 133–135</td><td>1.158</td><td>175</td><td>6.616</td></tr><tr><td>25R-3, 135–137</td><td>0.571</td><td>181</td><td>3.156</td></tr><tr><td>36R-5, 41–43</td><td>5.519</td><td>292</td><td>18.900</td></tr><tr><td colspan="4"><strong>Green sediment</strong></td></tr><tr><td>9R-1, 65–67</td><td>0.132</td><td>150</td><td>0.878</td></tr><tr><td>12R-5, 139–141</td><td>0.109</td><td>218</td><td>0.501</td></tr><tr><td>13R-1, 44–46</td><td>0.094</td><td>131</td><td>0.718</td></tr><tr><td>14R-3, 18–20</td><td>0.098</td><td>335</td><td>0.292</td></tr><tr><td>18R-3, 28–30</td><td>0.156</td><td>90</td><td>1.728</td></tr><tr><td>40R-3, 100–102</td><td>0.084</td><td>137</td><td>0.612</td></tr></tbody></table>

fractures in basalts of the lowermost part of Hole U1502B could significantly prevent pervasive alteration, which in turn would result in higher $κ$ values. In many other magnetic studies of basaltic flows, flow boundaries are marked by jumps in magnetic susceptibility (e.g., Cañón-Tapia, 2004; Boiron et al., 2013; Caballero-Miranda et al., 2016). Regardless of the origin of high-$κ$ intervals, these intervals most likely mark boundaries between effusive units with unit thickness ranging between ≈14 and 28 m and an average thickness of ≈19 ± 5 m (Figure F51). The variation of magnetic susceptibility (point measurement on section halves) versus chemical composition from pXRF (see **Igneous and metamorphic petrology**) shows no distinct correlation ($R² = 0.343$ for $Fe_2O_3$; Figure F52), suggesting that alteration may not be the primary control on $κ$.

NRM at room temperature and magnetic susceptibility from archive halves were used to compute the Koenigsberger ratio, which indicates the capacity of a rock to carry magnetic anomalies. The Koenigsberger ratio ($Q_n$) is defined as the ratio between the remaining magnetization and magnetization induced by the geomagnetic field (43,103.1 nT = 34.3 A/m at Site U1502 according to World Magnetic Model 2015; Chulliat et al., 2015):

$$Q_n = \frac{\text{NRM (A/m)}}{[\text{K (SI)}] \times \text{H (A/m)}].$$

The distribution of the Koenigsberger ratio (Figure F53) indicates that altered basalts may contribute more to magnetic anomalies ($Q_n = 1–10$) than fresher basalts ($Q_n = 0.001–1$).

The mean dip angle of magnetic foliation calculated for Hole U1502B basalts (43 discrete samples, most of which are anisotropic samples is 8° (at the 95% confidence level) and thus lower than expected considering the apparent dip of seismic reflectors at this site (see **Background and objectives**), but it is characterized by a large ellipse of confidence. It is not possible to determine if this dip angle is primary or was acquired during subsequent tilting. However, the degree of magnetic anisotropy ($P'$) of these basalts is overall very small: $P' < 1.02$ (Figure F54). Such low degrees of anisotropy are common in basaltic flows that rapidly cooled on gentle slopes with <10° of dip (e.g., Boiron et al., 2013).

The magnetic shape parameter ($T$) shows a general ellipsoid symmetry close to 0, as would be expected from a magnetostatic fabric carrier such as magnetite or titanomagnetite (Figure F54). The low magnetite content in the altered basalts combined with the contribution of mafic silicates results in weak anisotropy of mag-
Figure F47. Demagnetization plots of (A, B) discrete samples and (C, D) archive-half sections, Hole U1502A sedimentary rocks. Zijderveld plots: solid squares = declination, open squares = inclination. Stereographic plots: solid squares = positive (down) inclination, open squares = negative (up) inclination. Calculated ChRM (blue line; red squares = measurements used in calculation) using PCA is also shown. A. Reversal polarity, Subunit IB B. Normal polarity, Subunit IE. C. Normal polarity, Subunit IA. D. Normal polarity, Subunit IA. MAD = maximum angular deviation.

**A** 368-U1502A-14R-3W, 18-20 cm, 493.41 m

<figure><img src="image_343.png" /></figure>

**B** 368-U1502A-39R-1W, 65-67 cm, 730.26 m

<figure><img src="image_344.png" /></figure>

**C** 368-U1502A-9R-1A, 22.5 cm, 442.525 m, red

<figure><img src="image_345.png" /></figure>

**D** 368-U1502A-9R-1A, 105 cm, 443.25 m, green

<figure><img src="image_346.png" /></figure>

netic susceptibility (AMS). Hence, this type of volcanic material is unlikely to yield valuable AMS data for subsequent magma flow studies.

## Magnetostratigraphy

The general AF demagnetization behavior of sedimentary rocks shows that 80% of the NRM is removed with a field of 25 mT. However, the predominance of steep normal inclinations (≈75°) across all late Oligocene to late Miocene units (375.00–750.67 m CSF-A) indicates that this remanence is caused by a large magnetic drilling overprint (Figure F49). Notwithstanding their higher NRM, the drilling overprint is partially removed but still present in most of the archive sections (Figure F55). This steep overprint is so strong that we are unable to remove it with in-line AF on the SRM, making paleoinclination data suspicious and preventing any reliable magnetostratigraphy to be established for both Holes U1502A and

U1502B. This situation arises from (1) the use of the RCB coring system, which is known to impart a strong overprint (e.g., Richter et al., 2007), (2) the naturally weak NRM of sediments, and (3) the magnetically soft nature of the magnetic assemblage present in these sediments.

The drilling overprint also considerably affects basalt. In several intervals between 728 and 920.8 m CSF-A, magnetic inclination displays negative values (Figure F55). However, the only data validating this magnetic reversal hypothesis occurs in Cores 368-U1502B-9R and 10R in two discrete samples (9R-2, 79–81 cm, and 10R-3, 16–18 cm). In this interval (gray shaded zone in Figure F55), the drilling overprint is less pervasive and the two discrete samples document both a normal (soft) component and a reversed (hard) component. Should this reversal be confirmed, it would imply that magmatic activity at Site U1502 lasted long enough to record a magnetic polarity reversal.
Figure F48. Magnetization intensity vs. inclination showing effect of steep drilling overprint, Site U1502. Basalts have higher magnetization intensity than sediments. Green lines = expected normal and reversal inclination at present latitudes.

<figure><img src="image_347.png" /></figure>

Figure F49. Paleomagnetic measurements, Hole U1502A. AFD = AF demagnetization. Green lines = expected normal and reversal inclination at present latitudes.

<figure><img src="image_348.png" /></figure>
Figure F50. AF and thermal demagnetization plots of (A, C, D) discrete samples and (B) archive-half section, Hole U1502B basalts. Zijderveld plots: solid squares = declination, open squares = inclination. Stereographic plots: solid squares = positive (down) inclination, open squares = negative (up) inclination. Calculated ChRM (blue line; red squares = measurements used in calculation) using PCA is also shown. A, B. Normal polarity. C. Normal polarity, thermally demagnetized. D. Normal polarity, AF demagnetized. MAD = maximum angular deviation.

A 368-U1502B-6R-3W, 49-51 cm, 761.22 m

<figure><img src="image_349.png" /></figure>

B 368-U1502B-11R-1A, 108 cm, 797.38 m

<figure><img src="image_350.png" /></figure>

C 368-U1502B-23R-2W, 65-67 cm, 851.99 m

<figure><img src="image_351.png" /></figure>

D 368-U1502B-23R-2W, 65-67 cm, 851.99 m

<figure><img src="image_352.png" /></figure>
Figure F51. Magnetic susceptibility of basalts, Hole U1502B. Gray bands correspond to abrupt variations of magnetic susceptibility. These boundaries have different origins: in some cases they are very close to flow margins (e.g., 6R-2W, 65-66 cm; Figure F30A), whereas in other cases they correspond to more massive/isotropic, and therefore less altered, intervals of flows. Gravitational accumulation of magnetite toward flow bottoms could cause an increase in magnetic susceptibility as shown elsewhere in basalt flows (Caballero-Miranda et al., 2016). If correct, this provides an independent tool to interpret basaltic flow boundaries. Green dashed line = boundary between more altered basalts above and less altered basalts below. Inset: bimodal distribution of magnetic susceptibility.

<figure><img src="image_353.png" /></figure>

Figure F52. Fe₂O₃ vs. κ showing a gap in magnetic susceptibility between stratigraphic units, Hole U1502B. Green = 368-U1502B-2R-1 to 35R-1, black = 35R-2 to 37R-4.

<figure><img src="image_354.png" /></figure>

Figure F53. Koenigsberger ratio showing relatively modest potential contributions ($Q_n < 10$), Hole U1502B basalts.

<figure><img src="image_355.png" /></figure>
Figure F54. AMS data, Hole U1502B igneous rocks. A. Stereonet of AMS directions (lower hemisphere, equal-area projection). K₁ = maximum principal axis, K₂ = intermediate principal axis; K₃ = minimum principal axis. Confidence ellipses at 95% level are shown in corresponding color. Tensorial means are shown with larger symbols of same color. B. Degree of magnetic anisotropy (P') vs. magnetic susceptibility (Km). C. Magnetic shape parameter (T) vs. P'.

<figure><img src="image_356.png" /></figure>

<figure><img src="image_357.png" /></figure>

<figure><img src="image_358.png" /></figure>

Figure F55. Paleomagnetic measurements, Hole U1502B. Gray shaded area = interval of possible negative inclination. TD = thermal demagnetization, AFD = AF demagnetization.

<figure><img src="image_359.png" /></figure>
# Geochemistry

Hydrocarbon gases were not abundant at Site U1502, and TOC, nitrogen, and sulfur contents were low, likely reflecting the poor preservation of organic matter. Instances of high carbonate content were associated with siderite and hydrothermal alteration, with siderite comprising greater than 50% of rock mass in parts of lithostratigraphic Subunit ID. The interstitial water at Site U1502 is enriched in Ca, Mg, K, and alkalinity and depleted in Na, Cl, and Br. The linear decrease and increase of major anion and cation contents, respectively, imply the upward diffusion of fluid from the deep sediment. Sediments within lithostratigraphic Unit I have some geochemical similarity to hemipelagic and pelagic units from Site U1501, but Site U1502 sediments have distinctively higher iron, potassium, and clay contents, likely due to patches of alteration resulting from different processes acting over various different time-scales. Thus, the mechanisms and timing of iron enrichment do not need to be the same. Analyzed igneous rocks from lithostratigraphic Units II and VI (i.e., igneous lithologic Units 1 and 2) are all altered, and in general, samples from igneous Subunit 2b are altered to a lesser degree than those from igneous Subunit 2a.

## Headspace gas

Headspace gas samples were taken at a frequency of one sample per core or per 10 m of drilling advance for routine safety monitoring to Core 368-U1502B-16R. Because of the absence of methane in

previous samples and the nature of the igneous lithologies encountered, an operational decision was made to no longer collect headspace samples after this point. A small cluster of high methane values was encountered deeper than 650 m in an interval corresponding to a zone of intense siderite formation in lithostratigraphic Subunit ID, but at no point did hydrocarbon gas levels exceed the 5 ppmv detection limit. Traces of propene and butene were observed on chromatograms, but their occurrence was sporadic and not quantified.

## Bulk sediment organic geochemistry

### Lithostratigraphic Unit I

TOC, nitrogen, and sulfur levels were low in all samples collected from lithostratigraphic Unit I (TOC < 1 wt%, nitrogen < 0.1 wt%, and sulfur frequently less than detection limit) (Table T12; Figure F56). These levels likely represent the consequence of a bioturbated environment that was unfavorable for the preservation of high quantities of organic matter (Morris and Calvert, 1977). The low C/N ratios (C/N < 10) are consistent with organic carbon being derived from marine sources (Hedges et al., 1988) or with a very high level of thermal alteration.

Higher carbonate contents are found where greater quantities of siderite occur within sediments, such as in lithostratigraphic Subunits IB and ID. Siderite can affect the acid digestion-based assay used by IODP to estimate carbonate contents. In Figure F56, siderite-corrected values are shown as red circles (assuming 100% siderite) alongside calcite values (blue, assuming 100% calcite). Because TOC is determined by the difference between total carbon and carbonate carbon, the anomalously high value for TOC in Table T12

<caption>Table T12. Carbon, nitrogen, and sulfur, Site U1502. View table in PDF format. Download table in CSV format.</caption>

<figure><img src="image_360.png" /><figcaption>Figure F56. Methane, TOC and total sulfur, carbon/nitrogen ratio, and carbonate (blue = assays assuming 100% calcite, red = assays assuming 100% siderite), Site U1502. OB = overburden. TN = total nitrogen.</figcaption></figure>
Figure F57. Crossplot of organic geochemical parameters: carbonate vs. carbon/nitrogen ratio and total sulfur vs. TOC, Site U1502. Solid circles = Subunit IA, open triangles = Subunit IB, open circles = Subunit ID.

<figure><img src="image_361.png" /><figcaption>Table T13. Interstitial water analyses, Hole U1502A. View table in PDF format. Download table in CSV format.</figcaption></figure>

likely reflects underestimation of the amount of inorganic carbon present.

Crossplots (Figure F57) do not help discriminate different organic geochemical units on the basis of lithostratigraphic units. In most cases there is a cluster of values with one or two outliers, which could be a consequence of comparing similar depositional environments (e.g., the units are all the same from a geochemical perspective) or indicate that hydrothermal alteration has overprinted any variation by reducing organic carbon and nitrogen content to levels beneath which it can be accurately measured.

### Lithostratigraphic Units II–VI

Only sporadic samples were collected from lithostratigraphic Units II–VI. These units are not organic carbon rich, and carbonate content is mostly low. The few instances of high TOC often correspond to carbonate-rich intervals, many of which are Fe, Mn, and Mg rich and could represent types of carbonate that are underestimated by the carbonate assay employed. The low sulfur content for some of these intervals may be unexpected given the prominence of pyrite within many cores. However, the metal content of most sulfides is high; thus, pyrite has a relatively low sulfur content in comparison to the mass of iron.

## Raman and surface-enhanced Raman spectroscopy

Because of time and resource constraints, Raman measurements were not performed at this site.

### Interstitial water chemistry

We collected 19 whole-round samples (15–20 cm long) for interstitial water measurements in Hole U1502A. For Cores 2R through 33R, 15 cm long samples were squeezed for interstitial water and yielded ~15 mL per sample. For Cores 34R and 35R (15 cm long) and 37R (20 cm long), whole-round samples yielded <8 mL of

interstitial water per sample and even then only after squeezing for durations longer than 8 h. Interstitial water geochemical parameters are listed in Table T13 and presented by depth in Figure F58.

No samples were obtained from depths shallower than 376.39 m (Section 2R-1) because this section was not cored (see Operations); therefore, in the following discussion comparisons are made to geochemical data from Holes U1499A and U1501C to set the data in context. In Hole U1502A, alkalinity increases linearly with depth from 7.6 to 45.9 mM. The maximum alkalinity was measured at 625 m, and the remaining measurements are constant at ~45 mM. In Hole U1501C, the alkalinity is at least 10 times greater than that found at the equivalent depth in Hole U1502A. At the same time, pH in Hole U1502A is lower than in Holes U1501C and U1499A, ranging from 7.2 to 7.9. The pH value decreases sharply from 376 to 531 m and is 7.3 at the bottom of the hole. Sulfate concentrations range from 4.0 to 9.9 mM with an increase from Core 2R to Core 37R. The bottom water in Hole U1502A is thus enriched in alkalinity and sulfate and has a higher pH than both modern seawater and most other interstitial water obtained in the region.

The chlorinity, bromide, and sodium depth profiles are similar; there is a 10% decrease from Core 2R to Core 37R (Figure F58). The uppermost sample from Core 2R is similar to modern seawater (Cl = 569 mM; Br = 0.89 mM; Na = 478 mM), whereas the bottom sample from Core 37R has low values of Cl (509 mM), Br (0.79 mM), and Na (421 mM). The decreased chlorinity, bromide, and sodium are similar to the “freshwater” in interstitial water Zone 3 in Hole U1501C.

Contrary to the halide anions, Ca, Mg, and K concentrations increase with depth (Figure F58). The calcium concentration ranges from 11 to 32.7 mM; the maximum is about three times that of modern seawater (10.5 mM). K concentrations vary between 3.2 and 5.3 mM; the maximum concentration is twice that found at similar depth samples in Holes U1501C and U1499A.

Dissolved strontium concentrations range from 61.9 to 144 µM and decrease with increasing depth, in contrast to the chemically similar Ca and Mg (Ca, Mg, and Sr covary in Hole U1499A). Interstitial water Sr concentration is often controlled by the dissolution
<figure><img src="image_362.png" /><figcaption>Figure F58. Interstitial water alkalinity, major cations, and anions, Site U1502.</figcaption></figure>

of biogenic calcite. The minimum Sr concentration (61.9 µM) at 676 m is lower than modern seawater and corresponds to elevated Ca and Mg.

Concentrations of Fe, Si, and Li vary in a similar way and gener-
ally increase with depth. Dissolved Fe concentrations range from
1.68 to 102.3 µM in the 477–716 m interval. The maximum Fe
(102.3 µM) at 676 m corresponds to maximum values of Si, Mg, and
Ca. The abnormally high Fe is likely related to the dissolution of
mafic minerals within the basalt. Silica concentrations increase
from 250 to 1182 µM in Hole U1502A and are much higher than
both modern seawater (0–180 µM) and Hole U1499A (280–822
µM) contents. The dissolved Si profile reflects lithology and silicate
mineral diagenesis. Lithium concentrations increase linearly from
91 µM at 376 m to the maximum of 855 µM at 695 m, which is
higher than the Li concentration at Site U1499 (13.7–31.2 µM).

Dissolved Mn concentrations increase from 1.14 to 47.73 µM to
the bottom of the hole.

Bulk sediment and rock geochemistry

The following sections describe sediments and igneous rock
geochemistry based on ICP-AES analysis. Compositions are re-
ported in Table T14.

Lithostratigraphic Unit I

Site U1502 sediments have high and variable Fe₂O₃ contents (2.81–38.59 wt%) that are significantly higher than those at Site

<caption>Table T14. Major element oxide and trace element contents of sediment and whole-rock samples, Site U1502. [View table in PDF format. Download table in CSV format.]</caption>

U1501 (Figure F59). Manganese is also enhanced (MnO = 0.01–1.49 wt%), with the highest contents found in carbonate-rich intervals, likely a result of the presence of Mn within siderite-rich intervals. PCA was used to compare sediments at Sites U1501 and U1502 (Figure F60) and shows some overlap with the oozes and hemipelagic mudstones (lithostratigraphic Units I and II at Site U1501), indicating unaltered samples. However, the greatest difference between the sediment samples is the proportion of K, Na, and Al (PC1) and the proportion of Si (PC2) (see Table T15). Some of the samples from Site U1502 contain notably more K, Al, and Ti (as well as Fe, although this was not included in the PCA), although by no means all of them. Given the hydrothermal alteration observed at this site, the enrichment of these elements in some of the sediment samples could reflect a clay or carbonate zone within an epithelial hydrothermal system (a low-temperature zone of phyllosilicate and carbonate formation; Sillitoe and Hedenquist, 2003), an increased proportion of clays in a depositional environment far from siliciclastic sources (Heezen and Hollister, 1972), poor preservation of siliceous fossils, or a combination of these processes. Lesser Ca contents within carbonate intervals may be the result of hydrothermal alteration and replacement by Fe, Mn, and Mg.
<figure><img src="image_363.png" /><figcaption>Figure F60. PCA crossplot for the two components that explain the greatest variation within data (PC1 = proportion of potassium, sodium, and aluminum; PC2 = proportion of silica), Site U1502. Circles and ellipses = 95% confidence intervals.</figcaption></figure>

<figure><img src="image_364.png" /><figcaption>Table T15. Major element oxide variance, Site U1502. Download table in CSV format.</figcaption></figure>

<table><thead><tr><th>Major element oxide</th><th>PC1 43% variance explained</th><th>PC1 29% variance explained</th></tr></thead><tbody><tr><td>Al<sub>2</sub>O<sub>3</sub></td><td>-0.461184109</td><td>-0.208145574</td></tr><tr><td>CaO</td><td>0.468018816</td><td>-0.262202401</td></tr><tr><td>SiO<sub>2</sub></td><td>-0.351496063</td><td>0.568850519</td></tr><tr><td>Na<sub>2</sub>O</td><td>-0.139784587</td><td>0.271894389</td></tr><tr><td>K<sub>2</sub>O</td><td>-0.481527489</td><td>-0.066640626</td></tr><tr><td>TiO</td><td>-0.424884295</td><td>-0.394468828</td></tr><tr><td>MgO</td><td>-0.113061955</td><td>-0.57477058</td></tr></tbody></table>

## Lithostratigraphic Units II and VI (igneous lithologic Units 1 and 2)

Major element oxides and minor element concentrations were normalized to TiO₂, which is thought to be constant during aqueous alteration processes (Staudigel, 2014). To provide a frame of reference, the resulting element ratios are plotted next to samples from IODP Expedition 349 Sites U1431, U1433, and U1434 in Figure F61.

When considered as a depth profile, a significant boundary appears to separate the samples into two groups of basalts at around 797.9 m (interval 368-U1502B-11R-2, 18–20 cm). This interval roughly corresponds with the boundary between igneous Subunits 2a and 2b (~801.55 m). Samples from shallower than 797.9 m are depleted in Fe, Mg, and Mn compared to those from deeper than 797.9 m (797.9–920.8 m). Also, these elements reveal lower contents than those of samples from fresh/less altered basalts from Ex-
<figure><img src="image_365.png" /><figcaption>Figure F61. Ti-normalized major element oxide and minor element concentration profiles, Site U1502. Green circles = whole rock samples from igneous lithologic Unit 1, black solid circles = samples from igneous lithologic Unit 2, red dots = fresh/less altered basalt samples from Expedition 349 (Li et al., 2015).</figcaption></figure>

pedition 349 (Figure F61). This depletion is less pronounced with depth. The Site U1502 basalts recovered from shallower than 797.9 m are also slightly less enriched in Ca, Si, Al, and subordinate V.

Na and K vary mostly within the ranges observed for fresher basalts from the study area, except for the 727.9–821.5 m interval (Figure F61). Ba, Sr, and Z are generally enriched relative to the samples of fresh/less altered basalts from the study area, which reflects the presence of veins rather than demonstrating a primary geochemical feature. Overall, when normalized to TiO₂, there are differences in the degree of alteration between igneous Subunits 2a and 2b; rock samples from igneous Subunit 2b suffered less alteration than those from igneous Subunit 2a.

Acidic interstitial water was encountered in overlying sediments, and if taken as representative of water from deeper sections

could indicate the potential for metasomatic alteration. Fe, Mg, Mn, and Al could all be mobilized by the dissolution of Fe-, Mg- or Al-bearing silicates such as olivine, pyroxene, and plagioclase (Pirajno, 2009). The likely products of such alteration are chlorite, phyllol-silicates, and zeolites.

## Discussion

To gain insight into the nature of authigenic mineralization in Hole U1502A, a 1-D material balance was performed (e.g., results are returned as grams Fe₂O₃ per square centimeter). The net gain or loss in Fe₂O₃ was calculated by making a comparison to Site U1501 in terms of sediment, to Site U1500 with respect to the basalts, and to shallower samples from Site U1501 in the case of interstitial water. For a given unit, the 1-D concentration was calculated as
Figure F62. Ti-normalized major element oxide and minor element concentration profiles, Site U1502. References are from fresh/less altered basalt samples from Expedition 349 (Li et al., 2015). mst = mudstone.

<figure><img src="image_366.png" /></figure>

$$[Fe]_{\text{balance}} = [Fe]_{\text{U1502}} - [Fe]_{\text{ref}}$$

where $[Fe]_{\text{balance}}$ is the net gain or loss in grams Fe per square centimeter for a given unit, $[Fe]_{\text{U1502}}$ is the measured iron content of a unit (grams Fe per square centimeter) at Site U1502, and $[Fe]_{\text{ref}}$ is the expected Fe content based on the lithologic points of reference listed above.

Fe contents per unit area were obtained using

$$[Fe] = h \times \%Fe_2O_3 \times \rho,$$

where $\rho$ is the average moisture and density (MAD) density within a given interval (in grams per cubic centimeter), $h$ is the length of a given interval (in centimeters), and $\%Fe_2O_3$ is the Fe content of a given interval expressed as a fractional content from 0 to 1. The results are displayed in Figure F62.

Interstitial water present in lithostratigraphic Subunits IA–IC and in intervals with a high siderite content is greatly enriched in Fe. The base of the interval spanning lithostratigraphic Subunits IA–IC coincides with Fe-enriched interstitial water that is consistent with instances of very low porosity that likely indicate hydraulic barriers that bound the interstitial water body. On a 1-D basis, it would be difficult to account for the high Fe content within the interstitial water by dissolution of the basalts at the top of lithostratigraphic Unit VI because of the magnitude of Fe involved.

The basalts at the top of lithostratigraphic Unit VI (igneous lithologic Unit 2) are depleted in Fe but are not significantly de-

pleted in Fe at the bottom of the recovered volcanic section, implying not that these basalts are not altered or depleted relative to Ti or in terms of their mineralogy, but that relative to the area of the rock the net depletion in Fe is slight. The slightness of the depletion may reflect the presence of Fe within the basalts in the form of sulfides or veins that precipitated from water not enriched in dissolved Ti (Staudigel, 2014). The level of Fe enrichment in sediment immediately above the basement (lithostratigraphic Subunit ID) is of a similar magnitude to the loss of Fe from the shallower basalt units. Compared to the upper part of lithostratigraphic Unit I, it is more reasonable that Fe mineralization within the lower part is genetically linked to the Fe depletion in parts of lithostratigraphic Unit VI.

These considerations represent a 1-D approximation and thus do not enable implications for structure, sedimentary architecture, or the geological history of the site. Therefore, they would not apply if the Fe was released from basalts significantly before the deposition of lithostratigraphic Unit I or if the precipitation of Fe occurred only within a small area yet leached from a greater area.

## Physical properties

Physical properties were measured on cores from Holes U1502A (375–758.2 m) and U1502B (727.7–920.8 m). Magnetic susceptibility, gamma ray attenuation (GRA) bulk density, and P-wave velocity were measured on the whole-round cores using the Whole-Round Multisensor Logger (WRMSL) at a 2 cm interval. NGR was then measured on whole-round cores with the Natural
Gamma Radiation Logger (NGRL). Color reflectance and point magnetic susceptibility were measured on all archive-half sections. P-wave velocity was also measured along the x-axis with the P-wave caliper (PWC) system on working-half sections with liner and on discrete samples without liner. MAD measurements were performed on discrete samples taken at a sampling resolution of one every other section per core. Thermal conductivity measurements were performed on the third section of every core. Based on the variations of physical properties, four petrophysical (PP) units were distinguished at Site U1502 (Figure F63):

* PP Unit 1 (375–740 m in Hole U1502A),

* PP Unit 2 (740–750 m in Hole U1502A and 727.7–740 m in Hole U1502B),

* PP Unit 3 (740–808 m in Hole U1502B), and

* PP Unit 4 (808–920 m in Hole U1502B).

PP Unit 1 is characterized by slight increases with depth in bulk density (average = 2.1 g/cm³) and P-wave velocity (average = 1950 m/s) and NGR values of 50–70 counts/s. PP Unit 2 lies immediately above the basaltic basement and is defined in both holes by a variety of different lithostratigraphic units. PP Unit 2 in Hole U1502A is

marked by lower P-wave velocity and bulk density and higher NGR values than overlying PP Unit 1. The base of PP Unit 2 is marked by a sharp decrease in NGR in both holes (Figures F63, F64). PP Units 3 and 4 represent the basaltic basement and are characterized by relatively low NGR (<10 counts/s) and a wider range of P-wave velocities (3000–5000 m/s). The interface between PP Units 3 and 4 is marked by an abrupt increase in density, P-wave velocity, and thermal conductivity and corresponds to the boundary between igneous Subunits 2a and 2b (see **Igneous and metamorphic petrology**).

## Natural gamma radiation

In PP Unit 1, NGR counts generally range between 50 and 70 counts/s with two exceptions (Figure F63). The first is a decrease in NGR at ~600 m to ~30 counts/s. The second interval starts at ~710 m, where NGR begins to decrease with depth, reaching ~35 counts/s at ~730 m, close to the top of PP Unit 2. PP Unit 2 is characterized by high NGR (70 counts/s) that decreases sharply near the base of this unit.

PP Unit 3 is characterized by relatively low NGR (3–10 counts/s) that decreases slightly with depth (Figures F63, F64). At 735 m, close to the top of PP Unit 3, a spike in NGR to ~50 counts/s cor-

<figure><img src="image_367.png" /><figcaption>Figure F63. Physical properties for Holes U1502A and U1502B with petrophysical units. Logarithmic scale is used for magnetic susceptibility. Descriptions of variations in P-wave velocity and bulk density in text are based on discrete measurements of P-wave velocity (PWC) and bulk density (MAD). cps = counts per second.</figcaption></figure>
<figure><img src="image_368.png" /><figcaption>Figure F64. Physical properties measured, Hole U1502B. Logarithmic scale is used for magnetic susceptibility. Descriptions of variations in *P*-wave velocity and bulk density in text are based on discrete measurements of *P*-wave velocity (PWC) and bulk density (MAD). cps = counts per second.</figcaption></figure>

relates with high *P*-wave velocity. Numerous smaller excursions in NGR values are present between 760 and 825 m. In contrast, deeper than 825 m (PP Unit 4), NGR values show only small-scale variations and remain less than ~5 counts/s.

Concentrations of K, U, and Th were estimated using the NGR counts and bulk density measurements for Hole U1502A according to the methods by De Vleeschouwer et al. (2017). Overall, element concentrations have trends similar to the total NGR counts per second values (Figure F65).

## *P*-wave velocity

Descriptions of variations in *P*-wave velocity are based on measurements made with the PWC system because measurements on the whole-round cores are more scattered due to the space between the core and core liner. In PP Unit 1, *P*-wave velocities increase gradually from 1735 m/s at ~375 m to ~2000 m/s at 715 m (Figure F63). This trend is interrupted by a spike in velocity at ~600 m to ~2910 m/s. Another spike in velocity occurs in the lowermost section of Hole U1502A, where velocity increases to ~2500 m/s over a narrow depth range (715–725 m). Below this interval, velocity rapidly decreases to ~1995 m/s at 740 m, marking the top of PP Unit 2. At the base of PP Unit 2 (~750 m), *P*-wave velocity increases abruptly to 2810 m/s.

In contrast to Hole U1502A (375–750 m), where *P*-wave velocity is consistently <3000 m/s, velocities in Hole U1502B (727.7–920.8

m) all exceed 3000 m/s (Figure F64). Two main intervals were observed in Hole U1502B: a shallower interval at 740–795 m in basal-tic basement (PP Unit 3; see Lithostratigraphy) with velocities in the 3000–4000 m/s range, and a deeper interval (PP Unit 4) starting at ~800 m with velocities that generally exceed 4000 m/s.

## Thermal conductivity

Thermal conductivity within PP Unit 1 varies between ~0.98 and ~1.90 W/(m·K) (Figure F63). A slight decrease in values from 1.5 to 1.1 W/(m·K) was observed at the bottom of PP Unit 1. A gradual increase in PP Unit 3 (740–808 m) from 1.9 to 2.3 W/(m·K) is followed by a sharp increase to 2.7 W/(m·K) at the boundary with PP Unit 4. Thermal conductivity decreases to 1.6 W/(m·K) in the upper portion of PP Unit 4 (843 m) (Figure F64) and averages ~2.2 W/(m·K) to the bottom of the unit. Two elevated thermal conductivity measurements (~3.2 W/[m·K]) were observed at 848 and 857 m.

## Density and porosity

In general, bulk density determined from discrete samples agrees well with those obtained from GRA on the whole-round cores (Figure F66). However, GRA bulk density displays large variations throughout Holes U1502A and U1502B because of space between the core and core liner. Thus, the following descriptions of variations in bulk density are based on measurements from discrete samples. Bulk density increases slightly with depth in PP Unit 1,
Figure F65. NGR measurements, Hole U1502A. K, U, and Th concentrations are derived from NGR and GRA density measurements using method of De Vlee-schouwer et al. (2017).

<figure><img src="image_369.png" /></figure>

varying from 1.9 to 2.3 g/cm³. Porosity decreases from 45% to 35% with depth and shows a good negative correlation with bulk density. Grain density remains relatively constant at 2.8 g/cm³. At 725 m, bulk density increases from 2.1 to 2.3 g/cm³ and dry density increases from 1.7 to 1.9 g/cm³. A sharp shift toward lower values at the bottom of this interval (740 m) marks the boundary with PP Unit 2. Densities then decrease slightly with depth to the bottom of PP Unit 2.

Bulk density in PP Unit 3 varies between 2.1 and 2.6 g/cm but generally increases with depth (Figure F66). Porosity fluctuates between 5% and 35% but decreases overall downhole.

The boundary between PP Units 3 and 4 is marked by a sharp increase followed by a drop in bulk, dry, and grain densities from 2.5 to 2.7 g/cm³, 2.3 to 2.5 g/cm³, and 2.8 to 3.0 g/cm³, respectively. Grain, bulk, and dry densities in PP Unit 4 are slightly more variable, ranging from 2.7 to 3.0 g/cm³, 2.4 to 2.8 g/cm³, and 2.1 to 2.7 g/cm³, respectively. Bulk and dry densities increase slightly with depth throughout PP Unit 4.

## Color reflectance spectrophotometry

L* values in PP Unit 1 remain relatively constant with an average of 40 and abruptly increase to 60 at the onset of PP Unit 2 (Figure F67). Throughout PP Units 2–4, L* values fluctuate between 20 and 70 without any general trend. From 375 to 675 m, reflectance parameters a* and b* show relatively narrow ranges of ~0–5 and approximately –4 to 2, respectively. Toward the lower portion of PP Unit 1 (~675–740 m), a* and b* display high values of ~5–10 and ~0–10, respectively, corresponding to the interbedded brownish and reddish clay (see Lithostratigraphy). The beginning of PP Unit 2 is marked by a rapid decrease in a* and b* followed by an overall decreasing trend through PP Units 2–4.

## Magnetic susceptibility

Magnetic susceptibility varies between ~10 × 10⁻⁵ and 100 × 10⁻⁵ SI in the upper portion of PP Unit 1 without any general trend,

and the fluctuations are relatively muted and narrow to 5 × 10⁻⁵ to 40 × 10⁻⁵ SI between ~700 and 740 m (Figure F63). Throughout PP Units 3 and 4, magnetic susceptibility generally has a narrow range between ~10 × 10⁻⁵ and 80 × 10⁻⁵ SI (Figure F64), except for the intervals where it peaks to ~2000 × 10⁻⁵ to 4000 × 10⁻⁵ SI (~835–842, 855–855.4, 872–875, 896–903, 908–912, and 914–921 m).

## RGB color values

Red, green, blue (RGB) values are generally constant with depth in PP Unit 1 (Figure F68). A marked increase in RGB from 100 to 150 occurs in the lower portion of PP Unit 1 (~725 m) and is related to the presence of brownish nannofossil-rich clay in the sediment cores (see Lithostratigraphy). RGB decreases in PP Unit 2 (740 m), marking a transition to dark gray biosiliceous clay over this interval (see Lithostratigraphy). RGB increases in PP Units 3 and 4 to between 100 and 200.

## Discussion and summary

The trends of physical properties allow us to characterize four petrophysical units at Site U1502. PP Unit 1 (375–740 m) reflects nannofossil- and biosiliceous-rich clay lithologies (see Lithostratigraphy) and exhibits small-scale changes in NGR values between 50 and 70 counts/s and uniformly increasing P-wave velocities with depth that are <2500 m/s. PP Unit 1 sediments have porosities similar to those recorded at nearby Sites U1499 and U1500 in the same depth interval, suggesting the same compaction history. A significant increase in bulk density (up to 2.7 g/cm³) was observed in Core 368-U1502B-3R and is associated with strongly lithified limestone beds (Figure F69A). Variations of high and low magnetic susceptibility values correspond to sediment color changes in PP Unit 1 (Figure F69B) and may be controlled by the amount of pyrrhotite within the sediment (Tauxe, 2010; see Paleomagnetism).

PP Unit 2 (740–750 m) in Hole U1502A is characterized by lower P-wave velocities and bulk densities and higher gamma ray readings than the strata above. Low densities and P-wave velocities
Figure F66. GRA bulk density; MAD bulk density, dry density, and grain density; and porosity, Holes U1502A–U1502B. Note that descriptions of variations in bulk density in text are based on discrete measurements of bulk density (MAD). MAD porosity from Sites U1499 and U1500 are also plotted for comparison.

<figure><img src="image_370.png" /><figcaption>Figure F67. Sediment color reflectance parameters L*, a*, and b*, Holes U1502A and U1502B.</figcaption></figure>

<figure><img src="image_371.png" /><figcaption>Figure F68. RGB values, Holes U1502A and U1502B.</figcaption></figure>

suggest that PP Unit 2 is less compacted and highly saturated with pore water.

PP Units 3 and 4 represent the basaltic basement and are characterized by relatively low NGR values (<10 counts/s), high densities, and P-wave velocities that vary over a broad range (3000–6000 m/s) (Figure F64). The boundary between PP Units 3 and 4 is marked by an abrupt increase in physical properties (i.e., density, P-

<figure><img src="image_372.png" /></figure>

wave velocity, and thermal conductivity) and coincides with the change from igneous Subunit 2a to 2b (Figures F64, F65, F66; see Igneous and metamorphic petrology). Prominent peaks of magnetic susceptibility (up to 4000 × 10⁻⁵ SI) in PP Units 3 and 4 correspond to relatively less altered basalt. Reflectance L* from these basalt layers records the lowest values in PP Unit 4, reflecting the dark nature of the less altered basalts. Thermal conductivities in PP Units 3 and 4 are variable but with an overall increasing trend, in line with the decreasing porosities with depth (Figure F64).
Figure F69. Comparison of sediment physical properties with lithology. A. Bulk density and NGR (368-U1502B-3R). B. magnetic susceptibility (MS) (368-U1502A-12R).

<figure><img src="image_373.png" /></figure>

## Downhole measurements
Logging operations

Two downhole logging strings were run in Hole U1502B. The first string was the triple combo tool string with DSI, and the second was the VSI tool string (Figures F70, F71). The end of the 10% inch casing was at 723 m, and the end of the drill pipe was initially set at 747.3 m. The modified triple combo tool string was able to reach 875 m in the open hole (~46 m above the total depth of RCB coring), and data were collected from that depth. The VSI tool string could not pass into the open hole, and check shots were conducted inside the casing. A summary of the log data compared to lithostratigraphy is shown in Figure F72.

### Logging Run 1: triple combo with sonic tool

After coring to 920.8 m in Hole U1502B, a wiper trip was made to verify the condition of the hole, and the RCB drill bit was released at the bottom of the hole. The drillers noted a ledge that might have developed a few meters below the casing; thus, the end of the drill

Figure F70. Logging operations summary, Hole U1502B. Left: Run 1; triple combo with DSI. Right: Run 2; VSI. mbsl = meters below sea level.

<figure><img src="image_374.png" /></figure>

pipe was raised to 747.3 m (~24 m below the end of casing), which became the top of open-hole logging in Run 1 (Figure F70).

Rig-up for the first downhole logging run started at 0000 h (local time; UTC + 8 h) on 13 May 2017. A modified triple combo with a sonic tool string was lowered into the hole at 0320 h and reached a maximum depth of 875.3 m at 0655 h, ~45 m above the total cored depth of Hole U1502B. The first upward pass began at 0655 h and ended at 0710 h, reaching 785.3 m in the open hole. The second upward pass started at 0720 h and ended at 0835 h, reaching close to the seafloor. Heave conditions were excellent for the entire logging operation, so the active heave compensator was not used.

In the modified triple combo run, the tool string included the HNGS, HLDS, and DSI for acoustic velocity (Figure F71). The centralized tools were run in the lower part of the tool string so that it would be easier to lower the tool string past ledges than it would be with an eccentricized tool (see **Downhole measurements** in the Expedition 367/368 methods chapter [Sun et al., 2018a]). The triple combo tool string did not encounter obvious obstructions in the open hole below the end of the drill pipe.

### Logging Run 2: VSI tool

Rig-up for the second downhole logging run using a VSI tool string started at 1230 h on 13 May 2017. By 1430 h, the drill pipe was pulled up to ~259 m, which was still deeper than the original planned depth of 80 m for the end of the drill pipe. However, it was determined that this would not affect the overall check shot stations, and thus the drill pipe was not pulled up any further. The VSI tool string was deployed at 1520 h and reached the end of casing (723.0 m) at 1730 h. Several attempts were made to get the VSI tool string to pass into the open hole below the casing, but it appeared to
Figure F71. Tools used in downhole logging operations, Hole U1502B. A. Run 1: triple combo with DSI. B. Run 2: VSI. LEH-MT = logging equipment head-mud temperature, EDTC = Enhanced Digital Telemetry Cartridge.

<figure><img src="image_375.png" /></figure>

be hanging up on a ledge and was unable to pass into the open hole section. With very little daylight remaining, we decided to concentrate on getting as many check shots inside the cased hole as time permitted.

In all, eight stations were attempted, all in casing. The two deepest stations (at 715.17 and 695.20 m) are associated with large noise, suggesting possible poor mechanical coupling of the casing with the surrounding rock formation. During the first run with the triple combo tool string, we noted that the amplitude of the natural gamma dropped significantly at the depth interval of ~640–747 m. Therefore, we suspected that the rock formation behind the casing might have collapsed and the debris fallen to the bottom of the hole, resulting in poor casing/hole contact. The remaining upper six stations, located at 290.23, 350.28, 450.31, 550.09, 590.27, and 630.34 m, recorded good quality waveforms (Figure F73; Table T16).

During the VSI operation, the IODP protected species procedures were implemented. A soft ramp-up of the seismic source was performed to a maximum pressure of 2000 psi. No protected species were observed. The seismic source was deployed by the aft port

crane at 38 m from the moonpool and 7 m below the sea surface (Figure F74). The seismic source has a cluster of two generator-injectors, each with a volume of 250 inch³. A MP-24 hydrophone was deployed at 9 m below the sea surface. The sea condition was calm. The VSI tool string was combined with telemetry and gamma ray tools (Figure F71). The VSI tool string has a downhole sampling rate and recording length of 1.0 and 3000 ms, respectively. A total of 47 seismic source shots were made during the experiment, from which 29 downhole seismic traces were used and processed to identify waveforms and first arrivals. After the VSI experiment, the tool was brought back to the surface and rigged down at 2200 h on 13 May.

## Logging units

The triple combo tool string run in Hole U1502B allowed us to define eight logging units (Table T17), many of which are correlated with changes in lithostratigraphic units, lithology, and physical properties (Figure F72). Logging Units 1–5 are inside both the casing and drill pipe, Unit 6 is outside the casing but inside the drill pipe, and Units 7 and 8 are in the open hole. When the logging tool is inside the casing and/or drill pipe, only measurements of NGR and the associated K, Th, and U components are meaningful, together with the understanding that the measured values were highly attenuated by the casing and/or drill pipe.

### Logging Units 1–6: seafloor to 747.3 m

Logging Units 1–5 were defined primarily based on the characteristics of the measured NGR within the casing (Figure F72). In Logging Unit 1 (seafloor–435.0 m), the amplitude of NGR increases gradually with depth from ~150 to 350 m to a relatively constant value; the amplitude of NGR then decreases toward the bottom of logging Unit 1 at 435 m. The bottom part of this logging unit (~380–435 m) corresponds to a zone of relatively poor recovery in Hole U1502A.

In logging Unit 2 (435.0–510.0 m), NGR is greater than it is in the adjacent logging units. Logging Unit 2 coincides with the interval of good core recovery in Hole U1502A and corresponds to the lower portion of lithostratigraphic Subunit 1A (Figure F72; see Lithostratigraphy). Logging Unit 3 (510.0–640.0 m) has NGR values similar to the bottom part of logging Unit 1. This unit coincides with the poor core recovery interval and is interpreted to represent sandy layers.

Logging Unit 4 (640.0–685.0 m) is associated with a major reduction (>50%) in the amplitude of NGR compared to logging Unit 3. Logging Unit 5 (685.0–723.0 m) has even greater reduction in the amplitude of NGR compared to logging Unit 4. We hypothesize that the rock formation throughout logging Units 4 and 5 might have become unstable and collapsed behind the casing. Such failure could cause poor acoustic contact between the casing and the adjacent formation, as reflected in the significant noise in the waveforms of the two check shot stations (at 715.17 and 695.20 m).

Logging Unit 6 (723.0–747.3 m) is outside of the casing but still inside the drill pipe. This logging unit corresponds to lithostratigraphic Subunit IE (Figure F72; see Lithostratigraphy).

### Logging Units 7 and 8: 747.3–853.0 m

Logging Units 7 and 8 are both in the open hole, outside of the casing and drill pipe. Logging Unit 7 (747.3–806.0 m) has relatively large hole diameters of 12–14.5 inches, logging bulk densities of 1.7–2.7 g/cm³, and $V_p$ values of 2.8–4.0 km/s. Logging bulk density is more scattered than the core sample density from MAD labora-
<figure><img src="image_376.png" /><figcaption>Figure F72. Downhole logs, lithostratigraphic units, and logging units, Holes U1502A and U1502B. HSGR = total spectral gamma ray, HCGR = computed (U-free) gamma ray, MAD = moisture and density (discrete samples), VSP = vertical seismic profile, PWC = P-wave caliper. Downhole logs were taken in Hole U1502B, whereas MAD, PWC, lithology, and core recovery data are from both Holes U1502A and U1502B.</figcaption></figure>

tory measurements. The logging velocity values are consistent with the P-wave velocity of core samples. The top of logging Unit 7 corresponds to a major change in lithology from sediment to altered

basalt. Logging Unit 7 corresponds to lithostratigraphic Subunit VIA (Figure F72; see Lithostratigraphy).
Figure F73. Waveforms and first arrivals of six check shot stations of good data quality, Site U1502. Red arrows = first arrivals picked using Schlumberger software.

<figure><img src="image_377.png" /><figcaption>Table T16. Versatile Seismic Imager check shot stack summary, Hole U1502B. Two-way traveltime corrections were made by logging specialists at the Lamont-Doherty Earth Observatory and sent back to the ship. Download table in CSV format.</figcaption></figure>

<table><caption>Figure F74. Deployment scheme of seismic source used during VSI tool string logging run, Site U1502.</caption><thead><tr><td>Depth (m)</td><td>One-way traveltime (s)</td><td>Two-way traveltime after correction (s)</td></tr></thead><tbody><tr><td>290.225</td><td>2.67962599</td><td>5.17452002</td></tr><tr><td>350.278</td><td>2.71228576</td><td>5.21342325</td></tr><tr><td>450.306</td><td>2.76361561</td><td>5.27017117</td></tr><tr><td>550.092</td><td>2.81246138</td><td>5.32176971</td></tr><tr><td>590.265</td><td>2.83186769</td><td>5.3419137</td></tr><tr><td>630.339</td><td>2.85131097</td><td>5.36195803</td></tr></tbody></table>

<figure><img src="image_378.png" /></figure>

Logging Unit 8 (806.0–853.0 m) has a relatively stable hole diameter of ~11–12 inches. Logging density and $V_p$ of Unit 8 are consistent with those measured from core samples. Density and $V_p$ in

logging Unit 8 are greater than in logging Unit 7. Logging Unit 8 corresponds to the top part of lithostratigraphic Subunit VIB (Figure F72; see Lithostratigraphy).
<table><caption>Table T17. Logging units, Hole U1502B. Download table in CSV format.</caption><thead><tr><th rowspan="2">Logging unit</th><th colspan="2">Depth (m)</th></tr><tr><th>Top</th><th>Bottom</th></tr></thead><tbody><tr><td>1</td><td>0</td><td>435</td></tr><tr><td>2</td><td>435</td><td>510</td></tr><tr><td>3</td><td>510</td><td>640</td></tr><tr><td>4</td><td>640</td><td>685</td></tr><tr><td>5</td><td>685</td><td>723</td></tr><tr><td>6</td><td>723</td><td>750</td></tr><tr><td>7</td><td>750</td><td>800</td></tr><tr><td>8</td><td>800</td><td>853</td></tr></tbody></table>

## Correlation to seismic data

We conducted two sets of analyses to correlate the core measurements (in meters below seafloor) to the time-based seismic data at Site U1502. First, we analyzed core data and investigated correlation to the seismic reflection profile. Second, we constructed synthetic seismograms using Site U1502 core data and compared the results with the traces in seismic profiles. Preliminary calculations were done during the expedition using a combination of Matlab scripts and Petrel software.

### Velocity and density data and comparison with seismic reflectors

Whole-round core measurements, including P-wave velocity logger (PWL) and GRA density measurements, were conducted from 375.0 to 750.7 m in Hole U1502A and from 727.7 to 920.8 m in Hole U1502B (Figure F75). P-wave velocity was also measured along the x-axis with the PWC on working-half sections. MAD measurements were performed on discrete samples, one every other section. Sonic velocity and HLDS density measurements were obtained from downhole logging in Hole U1502B at 747.3–853.0 m (see Physical properties and Downhole measurements).

With the outliers filtered out, the whole-round PWL velocity and GRA density data are in good agreement with the PWC velocity and MAD density of core samples, respectively (Figure F75). Sonic velocities from downhole logging agree well with the PWC data, and the HLDS density from logging is also in good agreement with the MAD core density for intervals away from borehole enlargements (Figure F72; see Physical properties and Downhole measurements).

The VSI logging run yielded one-way traveltime (OWT) values at six depths: 290.23, 350.28, 450.31, 550.09, 590.27, and 630.34 m (Figure F75; see Downhole measurements). The corresponding interval velocities were calculated for six depth intervals, respectively: 0–290.23, 290.23–350.28, 350.28–450.31, 450.31–550.09, 550.09–590.27, and 590.27–630.34 m. $V_p$ and density noticeably increase at ~740 m, coinciding with the lithologic change from sediment to altered basalt (see Lithostratigraphy). We hypothesize that the

strongest amplitude in the seismic traces in Hole U1502A might correspond to the major changes in lithology and physical properties at 740 m and define a first reference point (RP1). To correlate the maximum amplitude in the seismic trace expressed in time (milliseconds) with the depth of 740 m, the calculated interval velocity for the 630.34–740.0 m interval should be 2.5 km/s. Another depth of noticeable changes in $V_p$ and density is at ~800.0–806.0 m, which we chose as a second reference point (RP2). For the 740.0–806.0 m interval, an interval velocity of 3.3 km/s was chosen, which corresponds to the average PWC and PWL velocity. Similarly, for the 806.0–920.0 m interval, an interval velocity of 4.0 km/s was calculated based on the average PWC and PWL $V_p$ values (Figure F75).

## Time-depth relationship

The check shot data from Hole U1502B and the PWC data from Holes U1502A and U1502B were used to calculate the time-depth relationship (TDR) model of seismic waves. OWT values were directly observed at the six check shot depths from 290.23 to 630.34 m, from which a TDR model was readily calculated (Figure F76). The TDR for the deeper sections was based on the interval velocity at intervals of 630.34–740.0, 740.0–806.0, and 806.0–920.8 m. The calculated TDR model was similar to that of Site U1500 for depths shallower than 740 m. For depths greater than 740 m, however, the Site U1502 TDR model corresponds to the relatively high velocities of altered basalts, whereas the Site U1500 model corresponds to the relatively low velocities of sediment layers.

Using the best-fitting TDR model for Site U1502 (Figure F76), we were able to convert “depth scale” to “timescale” for all core measurements. This conversion enabled us to display the core measurements directly on top of the time-based seismic sections, facilitating identification of correlated features, as illustrated in the example of Figure F77.

## Synthetic seismograms

To further illustrate the relationship between the core measurements at Site U1502 and the seismic reflection profile, the Seismic interpretation module in Petrel was used to compute synthetic seismograms based on the relevant core measurements (Figure F78). Synthetic traces were generated by convolving the reflectivity from the density of core samples after smoothing by a 20-point running window, by a smoothed-interval velocity model, and with the source wavelet that best fits the seismic data (Ricker 2; Figure F78).

Seismic impedance (SI) of a rock layer is the product of its average seismic velocity ($V_p$) and density ($\rho$) (i.e., $SI = V_p \times \rho$). The seismic reflection coefficient (RC), in turn, equals the change in the seismic impedance between two adjacent rock layers. Thus, we can expect greater amplitudes in the modeled seismogram for rock interfaces with larger values of RC. The resulting synthetic seismogram shows strong reflectors at 730 and 810 m, consistent with the observed range of seismic reflectors (Figure F78).
<figure><img src="image_379.png" /><figcaption>Figure F75. Comparison of a subset of physical properties (MS, velocity, and density) with seismic waveform (converted to depth scale using best-fitting TDR model) and seismic image (in timescale). Red circles = check shot points (top six) and reference points (bottom two). Vint = velocity for TDR.</figcaption></figure>
Figure F76. A. Site U1502 time-depth relation curve compared to Site U1500. TDR model for Site U1502 was constrained by check shot data at six depths (CS1–CS6; red circles). Red squares = reference points at 750.0 and 806.0 m (RP1 and RP2), coinciding with noticeable changes in density and velocity. B. Hole U1502A and U1502B PWC data and Hole U1502B logging $V_p$. Interval velocity model was derived using both check shot data (above 630.34 m) and average PWC and logging velocities (below 630.34 m).

<figure><img src="image_380.png" /></figure>
<figure><img src="image_381.png" /><figcaption>Figure F77. Seismic section at Site U1502 with lithostratigraphic units overlain at drill hole location. Also shown are selected core measurements converted to depth scale using best-fitting TDR model (density, MS, PWL and PWC, porosity, and NGR).</figcaption></figure>
<figure><img src="image_382.png" /><figcaption>Figure F78. Synthetic seismogram, Site U1502. Panels from left to right: smoothed velocity and MAD density used in the calculation, resultant reflection coefficient (RC), real seismic traces with synthetic seismogram inserted in the center, and seismic source function "Ricker 2" used in modeling. TVD = true vertical depth calculated by Petrel software.</figcaption></figure>

## References

Barron, J.A., 1985. Miocene to Holocene planktic diatoms. *In* Bolli, H.M., Saunders, J.B., and Perch-Nielsen, K. (Eds.), *Plankton Stratigraphy: Cambridge, United Kingdom (Cambridge University Press)*, 763–809.

Briais, A., Patriat, P., and Tapponnier, P., 1993. Updated interpretation of magnetic anomalies and seafloor spreading stages in the South China Sea:

implications for the Tertiary tectonics of Southeast Asia. *Journal of Geophysical Research: Solid Earth*, 98(B4):6299–6328.

https://doi.org/10.1029/92JB02280

Boiron, T., Bascou, J., Camps, P., Ferré, E.C., Maurice, C., Guy, B., Gerbe, M.-C., and Launeau, P., 2013. Internal structure of basalt flows: insights from magnetic and crystallographic fabrics of the La Palisse volcanics, French
massif central. *Geophysical Journal International*, 193(2):585–602.

https://doi.org/10.1093/gji/ggs115

Caballero-Miranda, C.I., Alva-Valdivia, L.M., González-Rangel, J.A., Gog-
itchaishvili, A., Urrutia-Fucugauchi, J., and Kontny, A., 2016. Vertical
AMS variation within basalt flow profiles from the Xitle volcano (Mexico)
as indicator of heterogeneous strain in lava flows. *Journal of Volcanology
and Geothermal Research*, 311:9–28.

https://doi.org/10.1016/j.jvolgeores.2016.01.003

Cañón-Tapia, E., 1994. Anisotropy of magnetic susceptibility parameters: guidelines for their rational selection. *Pure and Applied Geophysics*, 142(2):365–382. https://doi.org/10.1007/BF00879310

Chulliat, A., Macmillan, S., Alken, P., Beggan, C., Nair, M., Hamilton, B.,
Woods, A., Ridley, V., Maus, S., and Thomson, A., 2015. *The US/UK
World Magnetic Model for 2015–2020: Technical Report*. Boulder, CO
(NOAA National Geophysical Data Center).

https://doi.org/10.7289/V5TB14V7

Clark, C., and James, P., 2003. Hydrothermal brecciation due to fluid pressure
fluctuations: examples from the Olary Domain, South Australia. *Tectono-
physics*, 366(3–4):187–206.

https://doi.org/10.1016/S0040-1951(03)00095-7

Cullen, A., Vicenzi, E., and McBirney, A.R., 1989. Plagioclase-ultraphyric basalts of the Galapagos Archipelago. *Journal of Volcanology and Geothermal Research*, 37(3–4):325–337.

https://doi.org/10.1016/0377-0273(89)90087-5

Dean, W.E., Gardner, J.V., and Hemphill-Haley, E., 1989. Changes in redox conditions in deep-sea sediments of the subarctic North Pacific Ocean: possible evidence for the presence of North Pacific Deep Water. *Paleoceanography*, 4(6):639–653.

https://doi.org/10.1029/PA004i006p00639

De Vleeschouwer, D., Dunlea, A.G., Auer, G., Anderson, C.H., Brumsack, H., de Loach, A., Gurnis, M., et al., 2017. Quantifying K, U, and Th contents of marine sediments using shipboard natural gamma radiation spectra measured on DV JOIDES Resolution. *Geochemistry, Geophysics, Geosystems*, 18(3):1053–1064. https://doi.org/10.1002/2016GC006715

Ellwood, B.B., Balsam, W., Burkart, B., Long, G.J., and Buhl, M.L., 1986. Anomalous magnetic properties in rocks containing the mineral siderite: paleomagnetic implications. *Journal of Geophysical Research: Solid Earth*, 91(B12):12779–12790. https://doi.org/10.1029/JB091iB12p12779

Fink, J., 2017. Laboratory Simulations of Submarine Lava Flow Morphology: A Quarter Century of Progress [presented at the American Geophysical Union 2017 Chapman Conference, Hobart, Tasmania, Australia, 29 January–3 February 2017].

Flower, M.F.J., 1980. Accumulation of calcic plagioclase in ocean-ridge tholeiite: an indication of spreading rate? *Nature*, 287(5782):530–532.

https://doi.org/10.1038/287530a0

Gardner, J.V., Dean, W.E., Klise, D.K., and Baldauf, J.G., 1982. A climate-
related oxidizing event in deep-sea sediment from the Bering Sea. *Qua-
ternary Research*, 18(1): 91–107.

https://doi.org/10.1016/0033-5894(82)90023-0

Gregg, T.K.P., and Fink, J.H., 1995. Quantification of submarine lava-flow morphology through analog experiments. *Geology*, 23(1):73–76.

https://doi.org/10.1130/0091-7613(1995)023<0073:QOSLFM>2.3.C
O;2

Hedges, J.I., Clark, W.A., and Come, G.L., 1988. Organic matter sources to the water column and surficial sediments of a marine bay. *Limnology and Oceanography*, 33(5):1116–1136.

https://doi.org/10.4319/lo.1988.33.5.1116

Heezen, B.C., and Hollister, C.D., 1971. *The Face of the Deep*: New York (Oxford University Press USA).

Hirt, A., and Gehring, A.U., 1991. Thermal alteration of the magnetic mineralogy in ferruginous rocks. *Journal of Geophysical Research: Solid Earth*, 96(B6):9947–9953. https://doi.org/10.1029/91JB00573

Hu, S., Stephenson, A., and Appel, E., 2002. A study of gyroremanent magnetisation (GRM) and rotational remanent magnetisation (RRM) carried by greigite from lake sediments. *Geophysical Journal International*, 151(2):469–474. https://doi.org/10.1046/j.1365-246X.2002.01793.x

Jébrak, M., 1997. Hydrothermal breccias in vein-type ore deposits: a review of mechanisms, morphology and size distribution. *Ore Geology Reviews*, 12(3):111–134. https://doi.org/10.1016/S0169-1368(97)00009-7

Kaminski, M.A., and Gradstein, F.M., 2005. Atlas of Paleogene cosmopolitan deep-water agglutinated foraminifera. *Grzybowski Foundation Special Publication*, 10. http://www.foraminifera.eu/atlas.html

Kennish, M.J., and Lutz, R.A., 1998. Morphology and distribution of lava flows on mid-ocean ridges: a review. *Earth-Science Reviews*, 43(3–4):63–90. https://doi.org/10.1016/S0012-8252(98)00006-3

Kirschvink, J.L., 1980. The least-squares line and plane and the analysis of palaeomagnetic data. *Geophysical Journal of the Royal Astronomical Society*, 62(3):699–718.

https://doi.org/10.1111/j.1365-246X.1980.tb02601.x

Lange, A.E., Nielsen, R.L., Tepley, F.J., III, and Kent, A.J.R., 2013. The petro-
genesis of plagioclase-phyric basalts at mid-ocean ridges. *Geochemistry,
Geophysics, Geosystems*, 14(8):3282–3296.

https://doi.org/10.1002/ggge.20207

Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Sun, Z., Stock, J.M., Klaus, A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018. Site U1501. *In Sun, Z.*, Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program).

https://doi.org/10.14379/iodp.proc.367368.105.2018

Li, C.-F., Lin, J., Kulhanek, D.K., Williams, T., Bao, R., Briais, A., Brown, E.A., Chen, Y., Clift, P.D., Colwell, F.S., Dadd, K.A., Ding, W., Hernández-Almeida, I., Huang, X.-L., Hyun, S., Jiang, T., Koppers, A.A.P., Li, Q., Liu, C., Liu, Q., Liu, Z., Nagai, R.H., Peleo-Alampay, A., Su, X., Sun, Z., Tejada, M.L.G., Trinh, H.S., Yeh, Y.-C., Zhang, C., Zhang, F., Zhang, G.-L., and Zhao, X., 2015. Expedition 349 summary. *In* Li, C.-F., Lin, J., Kulhanek, D.K., and the Expedition 349 Scientists, *South China Sea Tectonics*. Proceedings of the International Ocean Discovery Program, 349: College Station, TX (International Ocean Discovery Program).

https://doi.org/10.14379/iodp.proc.349.101.2015

Lurcock, P.C., and Wilson, G.S., 2012. PuffinPlot: a versatile, user-friendly program for paleomagnetic analysis. *Geochemistry, Geophysics, Geosystems*, 13(6):Q06Z45. https://doi.org/10.1029/2012GC00498

McClinton, J.T., White, S.M., Colman, A., Rubin, K.H., and Sinton, J.M., 2014. The role of crystallinity and viscosity in the formation of submarine lava flow morphology. *Bulletin of Volcanology*, 76:854.

https://doi.org/10.1007/s00445-014-0854-2

Morris, R.J., and Calvert, S.E., 1977. Geochemical studies of organic-rich sed-
iments from the Namibian shelf: 1. The organic fractions. *In* Angel, M.
(Ed.), *Voyage of Discovery*: Oxford, United Kingdom (Pergamon Press),
647–665.

Pan, Y., Zhu, R., Banerjee, S.K., Gill, J., and Williams, Q., 2000. Rock magnetic properties related to thermal treatment of siderite: behavior and interpretation. *Journal of Geophysical Research: Solid Earth*, 105(B1):783–794.

https://doi.org/10.1029/1999JB900358

Pirajno, F., 2009. *Hydrothermal Processes and Mineral Systems*: Dordrecht, Netherlands (Springer). https://doi.org/10.1007/978-1-4020-8613-7

Reyes, A.G., 1990. Petrology of Philippine geothermal systems and the appli-
cation of alteration mineralogy to their assessment. *Journal of Volcanol-
ology and Geothermal Research*, 43(1–4):279–309.

https://doi.org/10.1016/0377-0273(90)90057-M

Richter, C., Acton, G., Endris, C., and Radsted, M., 2007. *Technical Note 34:*
*Handbook for Shipboard Paleomagnetists*. Ocean Drilling Program.

https://doi.org/10.2973/odp.tn.34.2007
Roberts, A.P., Chang, L., Rowan, C.J., Horng, C.-S., and Florindo, F., 2011. Magnetic properties of sedimentary greigite (Fe₃S₄): an update. *Reviews of Geophysics*, 49(1):RG1002. https://doi.org/10.1029/2010RG000336

Sillitoe, R.H., and Hedenquist, J.W., 2003. Linkages between volcanotectonic settings, ore-fluid compositions, and epithermal precious metal deposits. In Simmons, S.F., and Graham, I. (Eds.), *Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth*. Society of Economic Geologists Special Publication, 10:315-343.

Snowball, I.F., 1991. Magnetic hysteresis properties of greigite (Fe₃S₄) and a new occurrence in Holocene sediments from Swedish Lappland. *Physics of the Earth and Planetary Interiors*, 68(1-2):32-40.
https://doi.org/10.1016/0031-9201(91)90004-2

Soule, S.A., Fornari, D.J., Perfit, M.R., and Rubin, K.H., 2007. New insights into mid-ocean ridge volcanic processes from the 2005-2006 eruption of the East Pacific Rise, 9°46′N-9°56′N. *Geology*, 35(12):1079-1082.
https://doi.org/10.1130/G23924A.1

Staudigel, H., 2014. Chemical fluxes from hydrothermal alteration of the oceanic crust. In Rudnick, R.L. (Ed.), *Treatise on Geochemistry* (2nd edition) (Volume 4): *The Crust*. Holland, H.D., and Turekian, K.K. (Series Eds.): New York (Elsevier), 583-606.
https://doi.org/10.1016/B978-0-08-095975-7.00318-1

Stock, J.M., Sun, Z., Klaus, A., Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018. Site U1500. In Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program).
https://doi.org/10.14379/iodp.proc.367368.104.2018

Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018a. Expedition 367/368 methods. In Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program,

367/368: College Station, TX (International Ocean Discovery Program).
https://doi.org/10.14379/iodp.proc.367368.102.2018

Sun, Z., Stock, J., Jian, Z., McIntosh, K., Alvarez Zarikian, C.A., and Klaus, A., 2016. *Expedition 367/368 Scientific Prospectus: South China Sea Rifted Margin*. International Ocean Discovery Program.
https://doi.org/10.14379/iodp.sp.367368.2016

Sun, Z., Stock, J.M., Klaus, A., Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018b. Site U1499. In Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program).
https://doi.org/10.14379/iodp.proc.367368.103.2018

Tarling, D.H., and Hrouda, F., 1993. *The Magnetic Anisotropy of Rocks*: London (Chapman and Hall).

Tauxe, L., 2010. *Essentials of Paleomagnetism*: Berkeley, California (University of California Press).

Thompson, G.F., 1990. The anomalous demagnetization of pyrrhotite. *Geo-physical Journal International*, 103:425-430.

White, W.M., and Klein, E.M., 2014. Composition of the oceanic crust. In Rudnick, R.L. (Ed.), *Treatise on Geochemistry* (2nd edition) (Volume 4): *The Crust*. Holland, H.D., and Turekian, K.K. (Series Eds.): New York (Elsevier), 457-496.
https://doi.org/10.1016/B978-0-08-095975-7.00315-6

Wilson, T.R.S., Thomson, J., Hydes, D.J., Colley, S., Culkin, F., and Sørensen, J., 1986. Oxidation fronts in pelagic sediments: diagenetic formation of metal-rich layers. *Science*, 232(4753):972-975.
https://doi.org/10.1126/science.232.4753.972

Yan, P., Zhou, D., and Liu, Z., 2001. A crustal structure profile across the northern continental margin of the South China Sea. *Tectonophysics*, 338(1):1-21. https://doi.org/10.1016/S0040-1951(01)00062-2

Zhao, F., Alves, T.M., Wu, S., Li, W., Huuse, M., Mi, L., Sun, Q., and Ma, B., 2016. Prolonged postrift magmatism on highly extended crust of divergent continental margins (Baiyun Sag, South China Sea). *Earth and Planetary Science Letters*, 445:79-91.
https://doi.org/10.1016/j.epsl.2016.04.001

Zijderveld, J.D.A., 1967. AC demagnetization of rocks: analysis of results. In Collinson, D.W., Creer, K.M., and Runcorn, S.K. (Eds.), *Developments in Solid Earth Geophysics* (Volume 3): *Methods in Palaeomagnetism*: Amsterdam (Elsevier), 254-286.
https://doi.org/10.1016/B978-1-4832-2894-5.50049-5
https://doi.org/10.14379/iodp.proc.367368.107.2018

## Site U1503¹

<figure><img src="image_383.png" /></figure>

Check for updates

Hans Christian Larsen, Zhimin Jian, Carlos A. Alvarez Zarikian, Zhen Sun, Joann M. Stock, Adam Klaus, Jacopo Boaga, Stephen A. Bowden, Anne Briais, Yifeng Chen, Deniz Cukur, Kelsie A. Dadd, Weiwei Ding, Michael J. Dorais, Eric C. Ferré, Fabricio Ferreira, Akira Furusawa, Aaron J. Gewecke, Jessica L. Hinojosa, Tobias W. Höfig, Kan-Hsi Hsiung, Baoqi Huang, Enqing Huang, Xiao-Long Huang, Shijun Jiang, Haiyan Jin, Benjamin G. Johnson, Robert M. Kurzawski, Chao Lei, Baohua Li, Li Li, Yanping Li, Jian Lin, Chang Liu, Chuanlian Liu, Zhifei Liu, Antonio Luna, Claudia Lupi, Anders J. McCarthy, Geoffroy Mohn, Lachit Singh Ningthoujam, Michael Nirrengarten, Nobuaki Osono, David W. Peate, Patricia Persaud, Ning Qiu, Caroline M. Robinson, Sara Satolli, Isabel Sauermilch, Julie C. Schindlbeck, Steven M. Skinner, Susanne M. Straub, Xiang Su, Liyan Tian, Froukje M. van der Zwan, Shiming Wan, Huaichun Wu, Rong Xiang, Rajeev Yadav, Liang Yi, Cuimei Zhang, Jinchang Zhang, Yang Zhang, Ning Zhao, Guangfa Zhong, and Lifeng Zhong<fnref n="2" />

**Keywords:** International Ocean Discovery Program, IODP, JOIDES Resolution, Expedition 367, Expedition 368, Site U1503, northern South China Sea, continent-ocean transition zone, Ridge C, hyperextension, continental breakup, thinning, rifting, Cenozoic

## Site summary

### Background and objectives

International Ocean Discovery Program (IODP) Site U1503 (proposed Site SCSIII-9B; Sun at al., 2016) is located at 3868 meters below sea level (mbsl) near the top of the structural high named Ridge C (Figures F1, F2). Ridge C is the most seaward ridge of the three margin-parallel Ridges A, B, and C that characterize the lower continental slope underlain by thin (5-7 km) crust. Ridge C is believed to represent at least partly, if not fully, igneous crust and hence the completion of continental breakup along this margin segment of the northern South China Sea (SCS).

A key operational objective of Site U1503 was to sample the lowest ~300 m of sediments on top of basement to constrain the age and subsidence history of the crust at this location, the timing of normal faulting, and the environment of the early half-graben fill. The most important goal was to sample the igneous stratigraphy to

at least 100 m below the basement. Because of a rig floor equipment failure (drawworks), we abandoned the site after installing casing in Hole U1503A to 991.5 m.

Deep, representative sampling of the basaltic material at this site would have provided an important reference frame for breakup modeling. With an estimated 1640 m sediment thickness overlying basement, obtaining basement samples and log data at this site represented a challenging operation.

## Operations

To reach our objectives, we conducted operations in one hole (U1503A; 18°08.6300′N, 116°18.8456′E; 3868 mbsl). We successfully installed a reentry system and 991.5 m of 10¾ inch casing in Hole U1503A. However, we were unable to deepen the hole below the casing because of repeated breakdowns of the low clutch diaphragm in the drawworks and concerns that we did not have enough spares to last the remainder of the expedition. Without the drawworks' low

## Contents

1. Site summary

2. Background and objectives

3. Operations

6. References

<figure><img src="image_384.png" /></figure>

<footnote n="1">¹ Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Sun, Z., Stock, J.M., Klaus, A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018. Site U1503. *In Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, South China Sea Rifted Margin. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.367368.107.2018*</footnote>

<footnote n="2">² **Expedition 367/368 Scientists' addresses.**</footnote>

<footnote>MS 367368-107: Published 28 September 2018</footnote>

<footnote>This work is distributed under the **Creative Commons Attribution 4.0 International** (CC BY 4.0) license.</footnote>

<footnote>© BY</footnote>
<figure><img src="image_385.png" /><figcaption>Figure F1. Bathymetry and seismic lines, Site U1503.</figcaption></figure>

clutch, we could not conduct drilling operations deeper than 3400 m (water depth plus penetration depth). Because Site U1503 had a planned depth of 5695 m (water depth plus penetration depth), we abandoned Hole U1503A without addressing any scientific objectives. Hole U1503A remains cased and open for possible future occupation.

## Background and objectives

Site U1503 is located at 3867.7 mbl (Figures F1, F2) near the top of the structural high named Ridge C. Ridge C is the most seaward ridge of the three margin-parallel Ridges A, B, and C that characterize the lower continental slope underlain by thin (5–7 km) crust (Figures F3, F4). Ridge C is believed to represent at least partly, if not fully, igneous crust and hence the completion of continental breakup along this margin segment of the northern SCS.

Ridge C is a remarkably continuous basement ridge consisting of two ridges separated by seaward-dipping normal faults (Figures F4, F5, F6). The offset of the normal faults is as high as ~400 ms two-way traveltime (TWT; ~400 m), and faulting apparently was active shortly after the formation of the igneous crust. Only the oldest of the overlying sediments filling the half-graben basins seems to be offset by the faults, whereas the younger sediments filling in the majority of the accommodation space generated by normal faulting shows a draping (and compaction) structure over the tectonically generated basement relief. Ridge C is very linear and parallel to Ridge B (see the Site U1500 chapter [Stock et al., 2018]) and sea-floor spreading anomalies south of the ridge (Figure F2). The top of the ridge is also remarkably flat along strike.

One important objective of Site U1503 was to sample the lowermost ~300 m of sediments on top of the basement to constrain the age and subsidence history of the crust in this location, the timing of normal faulting, and the environment of the early half-graben fill. The most important goal was to sample the igneous stratigraphy to at least 100 m below top of basement.

<figure><img src="image_386.png" /><figcaption>Figure F2. Bathymetric maps showing Expedition 367/368 sites (stars), and (A) regional and (B) local coverage of multichannel seismic reflection data and OBS data. Thick blue and red lines are key seismic lines used for planning of the drilling transect. A. Magnetic isochrons (orange lines) from Briais et al. (1993). B. Magnetic picks (orange squares) from the same reference, extracted from the Seton et al. (2014) compilation. Chron labels for the picks correspond to the old edge of the normal polarity intervals (see Ogg et al. [2016] timescale for ages). Orange square = Leg 184 Site 1148, yellow squares = Expedition 349 Sites U1432 and U1435.</figcaption></figure>

Based on the breakup-related basaltic magmatism found at both Sites U1502 (Ridge A) and U1500 (Ridge B), we believe that this rifted margin, unlike magma-starved Iberia-type margins, experienced massive igneous activity during breakup and that highly extended subcontinental lithosphere within the continent-ocean transition (COT), if present at all, now is likely to be more or less covered by igneous rocks. By implication, the reading of the terminal stages of lithospheric thinning prior to final breakup needs to be gleaned from the detailed composition of the igneous products, such as continental crustal contamination of the magmas, depth of
Figure F3. Two-way traveltime to (A) basement (Tg reflector) and (B) T60 unconformity. Proposed drilling transect (thick black line) is approximately at the center of a margin segment bounded to the southwest by a transform fault. Northeastern boundary of margin segment is around Expedition 349 Site U1435. At this location, outer margin high (OMH) and Ridge A seem to coalesce, and Ridges B and C of the COT become indistinct toward the northeast within the next margin segment. Note that the OMH is slightly oblique to the more parallel Ridges A, B, and C.

<figure><img src="image_387.png" /></figure>

and shallowing of mantle melting with time, and mantle composi-
tion, including wetness and possible temperature anomalies. The
crust at Ridge C is interpreted to represent the end-member of the
transition from breakup igneous activity initially modified by an
overlying lid of continental lithosphere to accretion of igneous crust
unmodified by a preexisting lithosphere. Deep representative sam-

pling of the basaltic material at this site therefore forms an important reference frame for the modeling of breakup, and basement penetration must be sufficient to provide a representative sampling of igneous units. With an estimated 1640 m of sediment to basement, Site U1503 was a challenging operation. Unfortunately, following installation of 990 m of steel casing within the upper sediments, repeated breakdown of the drawworks' low clutch diaphragm (see **Operations**) prevented Expedition 368 from deploying a drill string longer than 3400 m (water depth plus penetration), and Site U1503 was left for possible occupation during a future expedition.

The 3400 m depth limit imposed by the flawed drawworks left very few drilling options for continuation of Expedition 368 operations. Almost all proposed sites are in water depths greater than 3400 m and/or had deep targets. One exception was Site U1504, which is located in 2823 mbl and has a shallow coring target (see **Background and objectives** in the Site U1504 chapter [Larsen et al., 2018]).

Operations

We conducted operations in one hole at Site U1503 (Table T1) with the objective of sampling the lowermost sediments and 150 m into the underlying basalt. Hole U1503A is located at 18°08.6300′N, 116°18.8456′E, at a water depth of 3867.7 m. We installed a reentry system and 991.5 m of casing (10¾ inch) in this hole. However, we were unable to continue deepening the hole because of mechanical problems with the drawworks, including a fourth clutch diaphragm failure and a breakdown of the aft shaft bearing on one of the El-magco brakes. The continuing failures of the low clutch diaphragm and the lack of available spares to last until the end of the expedition limited the operation of the ship’s drilling equipment to depths shallower than 3400 m. Therefore, Site U1503 was abandoned without achieving any scientific objectives. Despite this major setback to the expedition and the South China Sea Rifted Margin program, Hole U1503A remains as a legacy hole for future occupation.

Hole U1503A

The R/V *JOIDES Resolution* began the transit from Site U1502 to Site U1503 in dynamic positioning mode while tripping pipe. The mud skirt and reentry cone intended for Site U1503 were moved to the center of the moonpool and stationed on the moonpool doors. With the mud skirt and reentry cone secured, the thrusters were pulled, and the vessel was underway at full speed at 0806 h on 14 May 2017.

The vessel arrived on site at 0930 h on 14 May but was unable to occupy the site until 1130 h because the Chinese R/V *Shiyan 2*, operated by the South China Sea Institute of Oceanology, was conducting a seismic survey in the area. The *JOIDES Resolution* completed the 19.8 nmi voyage from Site U1502 in 8.6 h (7.2 h in dynamic positioning mode and 1.4 h at cruise speed).

Once on site, a seafloor positioning beacon was deployed and the crew made up the bottom-hole assembly (BHA) with a 14¾ inch bit to drill a hole without coring to 1000 m for installation of 10¾ inch casing. The bit was lowered through the reentry cone and base that was assembled and placed in the moonpool. The precision depth recorder (PDR) measurement taken upon arrival placed the seafloor at 3873.1 mbl. The bit was lowered to 3846 mbl, the top drive was picked up, and drilling in Hole U1503A started at 2010 h on 14 May. Drilling of the 14¾ inch diameter hole penetrated to a total depth of 902.8 m at 0100 h on 16 May. At this time, the rig
Figure F4. Deep crustal time-migrated seismic reflection data without and with interpretation. Note the rather thin lower crust (two layers) above a strong Mohorovičić seismic discontinuity (Moho) reflector that can be followed oceanward. Moho reflection is weak to absent seaward from around the interpreted COT. Wide-angle seismic data (Yan et al., 2001) confirm ~6 km thick ocean crust (OC) seaward of the COT. A large detachment fault ~150 km inland of the COT separates more stable crust landward from that of highly extended crust seaward. An OMH is a fairly consistent feature along this margin segment. Key seismic unconformities are shown in purple (T70; ~32 Ma breakup unconformity?) and blue (T60; ~23 Ma regional basin event). These ages are inferred from long-distance (>100 km) correlation of seismic unconformities with industry holes and ODP Leg 184 Site 1148 (T60); ages need confirmation by coring and are only tentative. Tg (black) is basement. Arrows = approximate positions of seafloor magnetic anomalies with chron numbers. Seismic data are from Line 04ec1555-08ec1555 (courtesy of Chinese National Offshore Oil Corporation [CNOOC]). Location of line is shown in Figure F1. CDP = common depth point. C11n, C10n, and C9n = approximate location of normal polarity magnetic isochrons. MSB = midslope basin.

<figure><img src="image_388.png" /></figure>

mechanic noticed a leak in the low clutch diaphragm, which had already been replaced once during Expedition 367 and twice during Expedition 368. It was decided to pull the bit to the surface before the diaphragm failed completely. The bit was raised to 97.6 m below the seafloor, and a reentry cone was deployed. The subsea camera was lowered to observe the landing orientation of the cone (Figure F7A). Once the camera was near the seafloor, the bit was pulled out of the hole, clearing the seafloor at 0750 h on 16 May. The bit tagged the internal aperture of the cone to verify a seafloor depth of 3867.7 m, 5.4 m higher than previously thought. The seafloor depth for this hole was adjusted to 908.2 m.

The bit was then pulled to 3846 m, and 215 ft of drill line was slipped and cut. The bit was pulled to the surface, clearing the rotary table at 1655 h. With no spare clutch diaphragms aboard and no possible delivery of spare diaphragms until ~20 May, it was decided to proceed with all operations that would not require use of the low drive of the drawworks.

The drill crew assembled 85 joints (989.3 m) of 10¾ inch casing and crossed over to a 16 inch casing hanger. During an attempt to engage the DrilQuip running tools, it was noted that the seal diameter on the casing hanger was too small. The crew modified the hanger to enlarge the seal diameter and engaged the running tool. The casing was then lowered into the moonpool and hung off the moonpool doors.

The crew then picked up the mud motor and underreamer, lowered the drilling assembly into the moonpool, and tested them. The rest of the drilling assembly was made up and lowered through the casing. The running tool was then reengaged into the casing hanger,

and the entire casing string and drilling assembly were lowered as far as possible (2379 m) without using the drawworks low clutch and then hung off the traveling blocks. The last operation was to make up three stands of knobbies and pump a pig through each stand to clean any rust. The stands were then racked back in the derrick for use when drilling in the casing.

The rig then went on downtime on Thursday, 18 May, at 0915 h until the replacement parts (low clutch diaphragms) were delivered to the ship at 2310 h on 20 May. The seagoing tug *Taikoo* arrived on site with three spare low clutch diaphragms. The tug stationed itself on the port aft side of the *JOIDES Resolution*. Supplies were off-loaded, the tug departed by 2320 h on 20 May, and the crew began replacing the low clutch diaphragm.

Repairs were completed by 0415 h on 21 May. The driller continued lowering the casing string from 2379 mbsl to the seafloor, filling the pipe with seawater every 25 stands. The subsea camera was deployed, and Hole U1503A was reentered at 1005 h on 21 May with the 10¾ inch casing string. The casing began taking weight at 342 m, so it was pulled back, the top drive was installed, and the mud motor, bit, and underreamer were used to drill in the casing to the bottom of the predrilled hole at 908.2 m. We continued drilling in the last 86.9 m of casing until we reached 991.5 m. The casing was released at 0915 h on 22 May, and the camera and drilling assembly were recovered. The mud motor and underreamer were flushed with freshwater and laid out. The casing running tool was dewatered and laid out.

The upper guide horn (UGH) was installed, and the drill crew began making up and deploying a rotary core barrel (RCB) BHA.
Figure F5. Seismic Line 08ec1555, primary seismic line for Site U1503 and crossing seismic line for proposed Site SCSIII-30A. Dashed lines = unconformities, red solid lines = faults.

<figure><img src="image_389.png" /></figure>

The bit was lowered to 1232 mbsl, at which depth the drill crew noticed a loud noise coming from the forward Elmagco brake of the drawworks. The aft bearing on the forward Elmagco brake had failed. The crew began working to replace the brake with a reconditioned unit that was loaded aboard in Hong Kong. This job was scheduled for the maintenance period following Expedition 368. We were lucky that the seas were calm because this typically can't be done at sea.

Repairs began at ~0530 h on 23 May. Because of the calm seas, the crew was able to make the required heavy crane lifts. The forward brake was disconnected and moved to the rig floor where the hub could be removed for use on the replacement. The 28,000 lb (12.7 MT) nonfunctioning brake was then transferred to the main deck, and the replacement brake was moved to the rig floor to have the hub fitted and to be installed on the forward end of the drawworks. The brake was moved into place, the electrical and water connections were made, and the shaft was aligned by 0600 h on 25 May.

Operations resumed with lowering the bit to 3819 mbsl at 1100 h on 25 May, at which time it was noticed that the low clutch diaphragm was leaking again. The diaphragm was replaced for the

Figure F6. Crossing seismic line (15ecLW5) for Site U1503. Dashed lines = unconformities.

<figure><img src="image_390.png" /></figure>

fourth time during this expedition (completed at 2100 h on 25 May). Because of the great water depth at the site's location combined with the planned deep penetration at Site U1503, extensive use of the low clutch would have been required to continue operations. Considering this fact, along with the number of diaphragm failures so far and a single remaining spare left on board the ship, it was determined that the safest action would be to terminate operations in Hole U1503A without reentering the cased hole. An optimistic delivery date for additional clutch diaphragm spares, if available, was estimated for 2 June. If Hole U1503A was reentered and an additional clutch diaphragm failed while coring below the casing, the remaining spare would be enough only to get the drill string out of the hole, and any failed operations in the open hole could have prevented a future reentry operation. With two more weeks left of the expedition, the very difficult decision was made to switch to operations at sites with shallower water depths and penetration that did not require the use of the low clutch while drilling or coring. Therefore, we decided to transit to Site U1504 (2816 mbsl) and continue operations there.

Before departing from Site U1503, the subsea camera was deployed to the seafloor to inspect the Hole U1503A reentry. The cone appeared to be 1–2 m below the seafloor but was clearly visible and expected to be available for reentry at a later date (Figures F7B, F8). Consequently, the site can be considered a legacy site for future expeditions. The camera was retrieved, the bit was recovered, and the rig was secured for transit. The thrusters were up at 0930 h on 26 May, and the vessel began the 42.5 nmi transit to Site U1504.
tion, TX (International Ocean Discovery Program).
https://doi.org/10.14379/iodp.proc.367368.108.2018

Stock, J.M., Sun, Z., Klaus, A., Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018. Site U1500. In Sun, Z., Jian,

Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the
Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program).
https://doi.org/10.14379/iodp.proc.367368.104.2018

Sun, Z., Stock, J., Jian, Z., McIntosh, K., Alvarez Zarikian, C.A., and Klaus, A., 2016. *Expedition 367/368 Scientific Prospectus: South China Sea Rifted Margin*. International Ocean Discovery Program.
https://doi.org/10.14379/iodp.sp.367368.2016

Yan, P., Zhou, D., and Liu, Z., 2001. A crustal structure profile across the northern continental margin of the South China Sea. *Tectonophysics*, 338(1):1–21. https://doi.org/10.1016/S0040-1951(01)00062-2
https://doi.org/10.14379/iodp.proc.367368.108.2018

## Site U1504¹

<figure><img src="image_391.png" /></figure>

Check for updates

Hans Christian Larsen, Zhimin Jian, Carlos A. Alvarez Zarikian, Zhen Sun, Joann M. Stock, Adam Klaus, Jacopo Boaga, Stephen A. Bowden, Anne Briais, Yifeng Chen, Deniz Cukur, Kelsie A. Dadd, Weiwei Ding, Michael J. Dorais, Eric C. Ferré, Fabricio Ferreira, Akira Furusawa, Aaron J. Gewecke, Jessica L. Hinojosa, Tobias W. Höfig, Kan-Hsi Hsiung, Baoqi Huang, Enqing Huang, Xiao-Long Huang, Shijun Jiang, Haiyan Jin, Benjamin G. Johnson, Robert M. Kurzawski, Chao Lei, Baohua Li, Li Li, Yanping Li, Jian Lin, Chang Liu, Chuanlian Liu, Zhifei Liu, Antonio Luna, Claudia Lupi, Anders J. McCarthy, Geoffroy Mohn, Lachit Singh Ningthoujam, Michael Nirrengarten, Nobuaki Osono, David W. Peate, Patricia Persaud, Ning Qiu, Caroline M. Robinson, Sara Satolli, Isabel Sauermilch, Julie C. Schindlbeck, Steven M. Skinner, Susanne M. Straub, Xiang Su, Liyan Tian, Froukje M. van der Zwan, Shiming Wan, Huaichun Wu, Rong Xiang, Rajeev Yadav, Liang Yi, Cuimei Zhang, Jinchang Zhang, Yang Zhang, Ning Zhao, Guangfa Zhong, and Lifeng Zhong<fnref n="2" />

**Keywords:** International Ocean Discovery Program, IODP, JOIDES Resolution, Expedition 367, Expedition 368, Site U1504, northern South China Sea, continent-ocean transition zone, hyperextension, continental breakup, thinning, rifting, greenschist, rifting, Eocene, larger benthic foraminifers, coral reef, rapid subsidence

## Site summary

### Background and objectives

Site U1504 (alternate Site SCSI-27A) was proposed during International Ocean Discovery Program (IODP) Expedition 367 and approved by the Environmental Protection and Safety Panel during Expedition 368 as an alternate site should there be time left following completion of the high-priority sites included in the *Scientific Prospectus* (Sun et al., 2016). Expedition 368 occupied Site U1504 because of the inability to continue drilling below the 990 m deep, cased Hole U1503A. Most of our remaining approved sites were in water deeper than 3400 m and/or required deep penetration. One exception was Site U1504, which is located at 2823 m below sea level (mbls) ~45 km east of Site U1501 on the outer margin high (OMH) and has a shallow coring target (Figures F1, F2).

## Contents

1. Site summary

5. Background and objectives

6. Operations

8. Lithostratigraphy

15. Igneous and metamorphic petrology

20. Structural geology

24. Biostratigraphy

27. Paleomagnetism

31. Geochemistry

33. Physical properties

36. Correlation to seismic data

40. References

## Operations

Two holes were drilled at Site U1504. In Hole U1504A (18°50.9199'N, 116°14.5397'E; 2816.6 mbls), we cored with the rotary core barrel (RCB) system from the seafloor to metamorphic basement at 134.8 m and then into basement to 165.5 m, recovering 52.77 m (32%). In Hole U1504B (~200 m southeast of Hole U1504A; 18°50.8213'N, 116°14.5978'E; 2843.0 mbls), we drilled without recovery to 88.2 m, cored with the RCB system to metamorphic basement at 107.9 m, and then cored into basement to 200 m, recovering 21.48 m (19%).

## Lithostratigraphy

The succession recovered at Site U1504 includes two sedimentary units (I and II) underlain by a metamorphic unit (III). Litho-

<figure><img src="image_392.png" /></figure>

<footnote n="1">¹ Larsen, H.C., Jian, Z., Alvarez Zarikian, C.A., Sun, Z., Stock, J.M., Klaus, A., Boaga, J., Bowden, S.A., Briais, A., Chen, Y., Cukur, D., Dadd, K.A., Ding, W., Dorais, M.J., Ferré, E.C., Ferreira, F., Furusawa, A., Gewecke, A.J., Hinojosa, J.L., Höfig, T.W., Hsiung, K.-H., Huang, B., Huang, E., Huang, X.-L., Jiang, S., Jin, H., Johnson, B.G., Kurzawski, R.M., Lei, C., Li, B., Li, L., Li, Y., Lin, J., Liu, C., Liu, C., Liu, Z., Luna, A., Lupi, C., McCarthy, A.J., Mohn, G., Ningthoujam, L.S., Nirrengarten, M., Osono, N., Peate, D.W., Persaud, P., Qui, N., Robinson, C.M., Satolli, S., Sauermilch, I., Schindlbeck, J.C., Skinner, S.M., Straub, S.M., Su, X., Tian, L., van der Zwan, F.M., Wan, S., Wu, H., Xiang, R., Yadav, R., Yi, L., Zhang, C., Zhang, J., Zhang, Y., Zhao, N., Zhong, G., and Zhong, L., 2018. Site U1504. In Sun, Z., Jian, Z., Stock, J.M., Larsen, H.C., Klaus, A., Alvarez Zarikian, C.A., and the Expedition 367/368 Scientists, *South China Sea Rifted Margin*. Proceedings of the International Ocean Discovery Program, 367/368: College Station, TX (International Ocean Discovery Program). https://doi.org/10.14379/iodp.proc.367368.108.2018</footnote>

<footnote n="2">² **Expedition 367/368 Scientists' addresses.**</footnote>
Figure F1. Bathymetry at Site U1504 and location of the site in relation to seismic lines. Note that the clear offset in bathymetry along the margin may indicate some transform offset within the deeper margin structure. Seismic Line 15ecLW6 crossing the structural high is shown in Figure F4.

<figure><img src="image_393.png" /></figure>

stratigraphic Unit I (early Miocene–Pleistocene) is dominated by nannofossil and foraminiferal ooze and minor nannofossil-rich clay and is divided into four subunits (IA–ID). Subunit IA (Hole U1504A = 17.50–46.59 m) is composed of nannofossil-rich clay, nannofossil ooze, nannofossil ooze with biogenic silica, and clay-rich nannofossil ooze. The color of the ooze gradually changes downhole from dark greenish gray to greenish gray and gray. Distinct intervals contain more silt. Thin silt laminations are often disrupted by bioturbation. Layering is contorted at the base of the subunit. Subunit IB (Hole U1504A = 46.59–59.73 m) is composed of light brown foraminifer-rich nannofossil ooze and pale brown nannofossil ooze with foraminifers. Subunit IC (Hole U1504A = 65.80–104.99 m) is composed of light greenish gray, greenish gray, light brownish gray, pale brown, and light gray nannofossil ooze with clay and greenish gray clay-rich nannofossil ooze intercalated with light gray nannofossil-rich foraminiferal ooze with clay, light brownish gray foraminifer-rich nannofossil ooze, and foraminifer-rich nannofossil ooze with clay. Subunit ID (Hole U1504A = 104.99–112.66 m; Hole U1504B = 88.20–92.31 m) is composed of light brown to yellowish brown nannofossil ooze with foraminifers intercalated with pink or brownish yellow foraminifer-rich nannofossil ooze. Unit II (Hole U1504A = 114.72–134.80 m; Hole U1504B = 97.90–107.91 m) is clast-supported, bioclast-rich limestone with larger benthic foraminifers (as large as 15 mm). Unit III (Hole U1504A = 136.40–163.70 m; Hole U1504B = 117.40–196.28 m) is composed of fine- to coarse-grained epidote-chlorite schist (Subunit IIIA) and calc-silicate schist (Subunit IIIB) with granofels clasts.

## Metamorphic petrology

We recovered 27 m (Hole U1504A) and 79 m (Hole U1504B) of a variety of mylonitic epidote-chlorite to calc-silicate schists containing granofels clasts. Recovery was <20% in metamorphic lithologic Unit 1. Lithologic changes with depth and between the two

Figure F2. Bathymetric maps showing Expedition 367/368 sites (stars), and (A) regional and (B) local coverage of multichannel seismic reflection data and OBS data. Thick blue and red lines are key seismic lines used for planning of the drilling transect. A. Magnetic isochrons (orange lines) from Briais et al. (1993). B. Magnetic picks (orange squares) from the same reference, extracted from the Seton et al. (2014) compilation. Chron labels for the picks correspond to the old edge of the normal polarity intervals (see Ogg et al. [2016] timescale for ages). Orange square = Leg 184 Site 1148, yellow squares = Expedition 349 Sites U1432 and U1435.

<figure><img src="image_394.png" /></figure>

holes were observed, and Unit 1 is divided based on a change in the predominant metamorphic lithology (Figure F3). Metamorphic lithologic Subunit 1a was recovered in both Holes U1504A and U1504B and consists of greenish gray, microcrystalline to coarse-grained epidote-chlorite schist with dark greenish gray granofels clasts (<10 cm) and epidote-chlorite breccia. The schists show a strong mylonitic foliation, whereas the clasts have generally isotropic textures showing local weakly developed foliation. Both the schist and the granofels clasts show, in general, a comparable mineral assemblage consisting of epidote, chlorite, feldspar, and quartz
Figure F3. Lithostratigraphic, petrographic/petrological, and structural observations at Site U1504 with strong focus on the metamorphic basement. A, B. Clast-supported limestone with large benthic foraminifers and coral fragments. C. Severely quartz-veined, angular clast of chlorite-epidote granofels enclosed in mylonitic foliation of a typical epidote-chlorite schist. D. Steep (~75°) widely anastomosing foliation in a quartz + feldspar-rich variety of the rock. Arrow = extensively stretched veined clast of chlorite-epidote granofels oriented parallel to the foliation. E. Epidote-chlorite schist with granofels clasts split parallel to the xy-plane of finite strain. Large granofels clast is crosscut by a shear band within the epidote-chlorite schist. Several sigma clasts are observed right above the shear band. Both deformation structures indicate normal sense of shear. A late, postkinematic vein crosscuts both the mylonitic foliation and the granofels clast. F. Tight foliation affected by crenulation cleavage in quartz-poor (i.e., more melanocratic) variety of the rock. G. Local occurrence of foliated impure calcite marble. H. Veined and altered aphyric mafic granofels clast (left side) and granofels clast interpreted as protolithic epidote vein (right side). Both clasts are internally undeformed but show synkinematic deformation together with the mylonitic fabric at their rims. I. Rigid (left) and strongly deformed (upper end) granofels clasts embedded in calc-silicate schist. Note the deflection of the mylonitic foliation at the clast boundaries. J. Long piece of core showing the characteristic morphology of the mylonitic foliation in calc-silicate schist of Subunit IIIB. Note the steep inclination and anastomosing geometry enclosing heterolithic clasts that are strongly elongated, folded, and oriented parallel to the foliation.

<figure><img src="image_395.png" /></figure>
± other phyllosilicates and accessory minerals and locally isolated subhedral dark minerals (pyroxene?). The granofels clasts are often crosscut by a network of quartz veins. Subunit 1b contains an alternation of calc-silicate schist and epidote-chlorite schist with granofels clasts, epidote and/or chlorite granofels, chlorite schist with epidote, epidote-chlorite gneiss, and minor marble. The lithologies are fine grained to coarse grained with mostly an inequigranular texture and some bimodal or equigranular textures, and the schistose sections show a strong mylonitic foliation. Several of the schists show reddish brown alteration, and late calcite veins crosscut the foliation of the deepest schists recovered (e.g., in Core 368-U1504A-20R). Minerals observed are epidote, quartz, chlorite, calcite, and feldspar. Subunit 1b contains numerous clasts with variable lithologies: cryptocrystalline granofels either with or without quartz and/or calcite veins, porphyritic rocks with tabular phenocrysts (possibly altered plagioclase), brecciated (epidote, calcite, and/or quartz) veins, and foliated granofels.

Handheld portable X-ray fluorescence (pXRF) analyses show an average mafic igneous composition for both the schist and the clasts but with a spread toward ultramafic (in Hole U1504B Subunit 1a) and more felsic compositions (in Subunit 1b). A clear change in sulfur contents occurs between Subunits 1a and 1b, the latter of which is below the detection limit. High Nb contents compared to mid-ocean-ridge basalt (MORB) indicate a potentially enriched source for the protolith.

Lithostratigraphic Unit III (metamorphic lithologic Unit 1) consists of mylonitic greenschist facies metamorphic rocks, as indicated by the presence of epidote and chlorite. We infer that the protolith most likely was breccia based on the different clast sizes and types and styles of deformation. The protolith likely had a mafic igneous composition. It is, however, unclear if this breccia with mafic clasts has a sedimentary origin (e.g., volcaniclastic) or represents a potentially hydrothermally altered form of basalt, gabbro, or ultramafic protolith. At present, the metamorphic evolution of this unit and its possible connection with the opening of the South China Sea (SCS) is unknown.

## Structural geology

Lithostratigraphic Unit I shows subhorizontal bedding and locally some minor possible slump folds, as observed in Section 368-U1504A-5R-1. Unit II is devoid of any deformation structures. The metamorphic basement at Site U1504 is formed by greenschist facies mylonitic epidote-chlorite schists and calc-silicate schists (Unit III). The rocks preserve distinct deformation structures resulting from changes in the mode of deformation (brittle/ductile); the modal amount of quartz + feldspar governing the (local) rheological behavior; the amount of accumulated strain; the occurrence of variably sized (up to decimeter scale), prekinematic, heterolithic clasts that mostly form rigid (i.e., internally undeformed) bodies within the ductile foliation; and/or superimposed multiple deformation phases. The steeply dipping (up to 75°) mylonitic foliation is characterized by distinct morphologies: (1) a widely spaced anastomosing foliation associated with leucocratic bands; (2) a tight, closely spaced, and often crenulated foliation associated with more melanocratic schist variations; and (3) a tight anastomosing foliation associated with calc-silicate schists enclosing angular to rounded heterolithic granofels clasts. Locally, the sense of shear is indicated by shear bands and sigma clasts. In metamorphic lithologic Subunit 1a (Figure F3), granofels clasts are consistently microcrystalline (chlorite + epidote) and dismembered by a network of mostly parallel and perpendicular quartz veins cutting each other. They often form apparently stretched, elongated bodies oriented parallel to the

foliation. Subunit 1b shows a wide range of prekinematic (mostly mafic) clasts enclosed by the foliation. Encountered varieties encompass highly phyric to aphyric rocks, as well as fragmented former (epidote/calcite/quartz) vein fillings. In some cases, such clasts show ductile internal deformation. Clast boundaries can be either distinctly sharp or diffuse, the latter indicating mechanical and/or chemical interactions with the surrounding foliation. Deformation in Subunit 1b is presumably strongly controlled by an inherited (protolithic) brecciated structure.

## Biostratigraphy

All core catcher samples at Site U1504 were analyzed for calcareous nannofossils, foraminifers, and diatoms. Additional samples were taken from intervals within the working-half core sections when necessary to refine the ages. Preservation of calcareous microfossils is good to very good in Cores 368-U1504A-2R through 12R, and 368-U1504B-2R and is poorly preserved (recrystallization) in the reefal limestone below Core 368-U1504A-12R. Planktonic foraminifers and calcareous nannofossils are abundant in Cores 368-U1504A-2R through 12R and 368-U1504B-2R and barren in Cores 368-U1504A-13R through 15R. Diatoms are present but poorly preserved in Cores 368-U1504A-2R through 5R and absent in rest of the samples. Twenty biostratigraphic datums were identified in a succession from the late Pleistocene to the early Miocene.

A possible hiatus between the early Pleistocene and late Miocene was determined within Core 368-U1504A-6R by abrupt planktonic foraminifer and nannofossil assemblage changes. Sedimentation rates at this site varied from ~6 mm/ky in the early middle Miocene to ~22 mm/ky during the Pleistocene.

Cores 368-U1504A-13R through 15R are mainly composed of reefal and larger benthic foraminiferal limestone. Thin section examination of these carbonate rocks indicates the presence of abundant *Nummulites*, suggesting an Eocene age. The abundant larger benthic foraminifers indicate deposition in a warm shallow-marine environment. In contrast, the abundant planktonic foraminifers and calcareous nannofossils in the carbonate ooze in the overlying sedimentary section (Cores 2R through 12R) indicate a deepwater environment since the early Miocene.

## Paleomagnetism

Most of the discrete sediment samples analyzed show an initial soft magnetic behavior attributed to titanomagnetite followed by a gyroremanent magnetization (GRM) greater than 70 mT attributed to greigite, a mineral already identified in Hole U1501C. Consistently steep normal inclinations (~60°) across all sedimentary units indicate a significant drilling overprint that is removed by alternating field (AF) demagnetization up to 15 mT. A succession of nine normal and eight reversed polarities are defined, particularly in the upper part of the cores (Figure F29).

Discrete samples show low $κ$ values, a moderate degree of magnetic anisotropy, and strongly oblate symmetry, probably of depositional origin. This subhorizontal planar fabric indicates deposition in a calm, pelagic environment with moderate traction.

Natural remanent magnetization (NRM) intensities of metamorphic rocks, measured on the superconducting rock magnetometer (SRM) and on four discrete cubes, are relatively weak and indicate a complex magnetic assemblage composed of magnetite and hematite. The anisotropy of magnetic susceptibility (AMS) fabrics consistently dip steeply and show a moderate to high degree of magnetic anisotropy, oblate symmetries, and oblique magnetic lineations, possibly indicating an oblique motion within this high-strain shear zone.
## Geochemistry

Hydrocarbon gases were not detected above background levels at Site U1504, and total organic carbon (TOC), total nitrogen (TN), and total sulfur (TS) contents were low (<0.5 wt%). Instances of high carbonate content were associated with bioclast-rich and clast-supported limestone. The upper part of the sediment was not cored. Interstitial water data from the cored sediments are broadly comparable to other sites.

## Physical properties

Four petrophysical (PP) units are identified at Site U1504 according to variations in the core physical properties. Core material in PP Units 1 and 2 consists of soft sediments. In general, with respect to Unit 2, Unit 1 has a higher average natural gamma radiation (NGR) count (~30 versus 20 counts/s), a weaker color reflectance (mean L* of ~40 versus 50), and a lower average RGB value (100 versus 150). These changes in physical properties between Units 1 and 2 correspond to a switch in sediment composition from nannofossil-rich clay to nannofossil ooze. A general increase in bulk (from ~1.4 to 1.8 g/cm³) and dry (from ~0.7 to 1.2 g/cm³) densities and a downhole decrease in porosity (from 75% to 56%) in Units 1 and 2 mainly indicate sediment compaction with time. The boundary between Units 1 and 2 is marked by a relatively rapid increase in P-wave velocity and bulk density and a rapid decrease in porosity compared with the general trends mentioned above. This physical property boundary corresponds to an unconformity in the seismic profile and to a hiatus confirmed by the biostratigraphic data.

P-wave velocity and bulk density increase from ~3800 to 5000 m/s and from ~2.4 to 2.9 g/cm³, respectively, within Unit 3, which corresponds to a transition in lithology from coral-rich limestone to clast-supported limestone. Coral-rich limestone in the upper portion of Unit 3 is also associated with a moderately high porosity of ~23%. Unit 4 is characterized by uniformly high velocity of ~5500 m/s, high bulk density of ~2.9 g/cm³, and low porosity of ~3%, which correspond to the epidote-chlorite schist. A large increase in both velocity and density at the Unit 2/3 and Unit 3/4 boundaries corresponds to strong reflectors in the seismic data. The gradual downhole increase in thermal conductivity values at shallow depths is likely due to progressive compaction of the sediments.

## Background and objectives

Site U1504 was proposed together with other relatively shallow penetration sites during Expedition 367. These sites were approved by the Environmental Protection and Safety Panel and IODP as alternate sites for Expedition 368 should there be time left following completion of the high-priority sites included in the *Scientific Prospectus*. Expedition 368 occupied Site U1504 because of the inability to continue drilling below the 990 m deep, cased Hole U1503A (see **Operations** in the Site U1503 chapter [Larsen et al., 2018b]). The depth limit of 3400 m (water depth plus penetration) imposed by the flawed drawworks left very few drilling options for continua-

Figure F4. Seismic Profile 15ecLW6 with location of Holes U1504A and U1504B. Fault structures and basement location are complex to interpret. Drill holes were placed in the most simple, well-defined locations with a high probability to reach crystalline basement. Drill site choice was constrained by seabed communications cables in the area (to the north). Basement at drill site is interpreted to be at the strong (second) reflector at ~3940 ms TWT. Distance between Holes U1504A and U1504B is ~200 m. CDP = common depth point. Dashed lines = unconformities, red solid line = fault.

<figure><img src="image_396.png" /></figure>

tion of Expedition 368 operations because almost all proposed sites are in water depths greater than 3400 m and/or require deep penetration. One exception was Site U1504, which is located in ~2830 m water depth and has a shallow coring target (Figure F2).

Site U1504 is located near the top of a structural high (Figures F1, F2, F4) that can be interpreted as an easterly extension of the OMH. The OMH was drilled at Sites U1501 and U1505 (Figure F5). At Site U1501, acoustic basement at ~600 m consists of highly lithified sandstones and conglomerate likely of pre-Cenozoic age (see the Site U1501 chapter [Larsen et al., 2018a]). Site U1504 is ~50 km east of Site U1501 (Figure F2), and the OMH at Site U1504 is much narrower and bounded to the north (landward) by a major normal fault. The OMH appears faulted (complex) to the south (seaward) and lacks the small rift basins that formed an important coring target at Sites U1501 and U1505 (Figure F5). These basins may, however, be present south (seaward) of Site U1504, where the top of the basement rapidly deepens in parallel with basinward crustal thinning. Site U1504 may be located within a transition zone into another (eastern) margin segment with a different rifting structure (Figure F1).

Site U1504 was designed to sample the basement below the thin (~120 m) layers of sediments covering this basement high (Figure F4). The specific goal was to find out how deeply tectonic exhumation during breakup might have reached into the lithosphere, or alternatively, whether the basement would be of igneous origin and related to breakup, similar to Site U1502. There was no predrilling interpretation of the seismic stratigraphy. The site is located in a place with a strong basement reflector with a reasonable horizontal extension. Basement appears to be heavily faulted both north and south of the drill site.
Figure F5. TWT to (A) basement (Tg reflector) and (B) T60 unconformity. Proposed drilling transect (thick black line) is approximately at the center of a margin segment bounded to the southwest by a transform fault. Northeastern boundary of margin segment around Expedition 349 Site U1435. At this location, OMH and Ridge A seem to coalesce, and Ridges B and C of the continuous-ocean transition (COT) become indistinct toward the northeast within the next margin segment. Note that the OMH is slightly oblique to the more parallel Ridges A, B, and C.

<figure><img src="image_397.png" /></figure>

## Operations

We drilled two holes at Site U1504 (Table T1). Hole U1504A is located at 18°50.9199′N, 116°14.5397′E, in a water depth of 2816.6

m. In this hole, we cored with the RCB system from the seafloor to 165.5 m, recovering 52.77 m (32%) of sedimentary and metamorphic rocks. Hole U1504B is located ~200 southeast of Hole U1504A at 18°50.8213′N, 116°14.5978′E, in a water depth of 2843.0 m. In this hole, we drilled without recovery to 88.2 m and then cored with the RCB system over a 111.8 m interval, recovering 21.48 m (19%) of mostly metamorphic rock.

## Transit to Site U1504

The R/V JOIDES Resolution began the transit from Site U1503 to Site U1504 at 0948 h on 26 May 2017 and arrived on site at 1400 h the same day. The thrusters were lowered, and the vessel was switched to dynamic positioning mode at 1432 h, beginning operations at Site U1504. An RCB bottom-hole assembly (BHA) was made up and deployed to 1720 mbsl before failure of the pipe spinner on the iron roughneck. The spinner was replaced, and the crew continued to lower the pipe to 2776 mbsl. The subsea camera was deployed, and the bit was placed at 2813 mbsl for a short seafloor survey to verify that no seafloor cables were in the area. The seafloor was tagged after the survey, determining a water depth of 2816.6 m. The camera was retrieved, and the top drive was picked up in preparation for starting Hole U1504A.

### Hole U1504A

Coring in Hole U1504A started at 0215 h, and the bit reached the sediment/basement contact at 136.4 m (at 1455 h on 27 May 2017). Coring continued to 165.5 m, at which depth erratic torque and overpull caused termination of coring operations at 1100 h on 28 May. The bit was recovered to the surface, clearing the seafloor at 1220 h and the rotary table at 1740 h. The rig floor was secured for the 22 nmi transit to Site U1505 by 1825 h on 28 May, temporarily ending operations at Site U1504.

Total time spent in Hole U1504A was 50.5 h (2.1 days). A total of 21 RCB cores were taken over a 165.5 m interval of loosely to well-consolidated sedimentary rock and metamorphic rocks with 52.77 m of recovery (32%).

## Return to Site U1504

We returned to Site U1504 after completing operations at Site U1505. The 22.2 nmi transit from Site U1505 to Site U1504 was completed at 0048 h on 03 June 2017. The thrusters were lowered, and the heading was controlled at 0357 h, beginning operations at Site U1504.

### Hole U1504B

An RCB BHA was made up, and core barrels were spaced out. A center bit was installed, and the drill string was lowered to 2776 m. The subsea camera was deployed to the bit, and the drill string was hung off the elevator stool. The low clutch was then dismantled, and the diaphragm was replaced for the fifth time. The diaphragm failed during operations at Site U1505 but was not replaced there because it was not necessary for operations at that site and the rig crew recommended installing the new diaphragm at the seafloor in Hole U1504B in case it was necessary during operations there. The replacement took 7.75 h. Once completed, the top drive was picked up and the bit was lowered to the seafloor.

Seafloor was tagged at 2843.0 mbsl. The subsea camera was recovered to the moonpool, and Hole U1504B was spudded at 2250 h on 4 June 2017. The hole was advanced without coring to 88.2 m.
<table><caption>Table T1. Site U1504 core summary. DRF = drilling depth below rig floor, DSF = drilling depth below seafloor. NA = not applicable. Core type: R = rotary core barrel, numeric core type = drilled interval. Download table in CSV format.</caption><thead><tr><th colspan="3">Hole U1504A</th><th colspan="3">Hole U1504B</th><th rowspan="2">Time on deck UTC (h)</th><th rowspan="2">Time to cut core (min)</th><th rowspan="2">Core barrel</th><th rowspan="2">Mud pumped (bbl)</th></tr><tr><th>Core</th><th>Type</th><th>Top of interval DSF (m)</th><th>Bottom of interval DSF (m)</th><th>Interval advanced (m)</th><th>Core recovered length (m)</th><th>Core curated (m)</th><th>Recovery (%)</th></tr></thead><tbody><tr><td colspan="10"><strong>368-U1504A</strong></td></tr><tr><td>1</td><td>R</td><td>0.0</td><td>7.8</td><td>7.8</td><td>0.00</td><td></td><td>0</td><td>5/26/2017 19:50</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>2</td><td>R</td><td>7.8</td><td>17.5</td><td>9.7</td><td>0.05</td><td>0.05</td><td>1</td><td>5/26/2017 20:35</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>3</td><td>R</td><td>17.5</td><td>27.2</td><td>9.7</td><td>3.85</td><td>3.85</td><td>40</td><td>5/26/2017 21:10</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>4</td><td>R</td><td>27.2</td><td>36.9</td><td>9.7</td><td>4.55</td><td>4.55</td><td>47</td><td>5/26/2017 22:10</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>5</td><td>R</td><td>36.9</td><td>46.5</td><td>9.6</td><td>1.63</td><td>1.63</td><td>17</td><td>5/26/2017 22:45</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>6</td><td>R</td><td>46.5</td><td>56.1</td><td>9.6</td><td>5.63</td><td>5.63</td><td>59</td><td>5/26/2017 23:30</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>7</td><td>R</td><td>56.1</td><td>65.8</td><td>9.7</td><td>3.68</td><td>3.68</td><td>38</td><td>5/27/2017 00:15</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>8</td><td>R</td><td>65.8</td><td>75.5</td><td>9.7</td><td>7.76</td><td>7.76</td><td>80</td><td>5/27/2017 01:10</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>9</td><td>R</td><td>75.5</td><td>85.2</td><td>9.7</td><td>2.81</td><td>2.81</td><td>29</td><td>5/27/2017 01:45</td><td>5</td><td>Nonmagnetic</td></tr><tr><td>10</td><td>R</td><td>85.2</td><td>95.0</td><td>9.8</td><td>8.76</td><td>8.76</td><td>89</td><td>5/27/2017 02:40</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>11</td><td>R</td><td>95.0</td><td>104.8</td><td>9.8</td><td></td><td></td><td></td><td>5/27/2017 03:35</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>12</td><td>R</td><td>104.8</td><td>114.7</td><td>9.9</td><td>7.86</td><td>7.86</td><td>79</td><td>5/27/2017 04:20</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>13</td><td>R</td><td>114.7</td><td>124.5</td><td>9.8</td><td>0.07</td><td>0.10</td><td>1</td><td>5/27/2017 05:10</td><td>15</td><td>Nonmagnetic</td></tr><tr><td>14</td><td>R</td><td>124.5</td><td>134.2</td><td>9.7</td><td>0.35</td><td>0.38</td><td>4</td><td>5/27/2017 05:55</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>15</td><td>R</td><td>134.2</td><td>136.4</td><td>2.2</td><td>0.66</td><td>0.66</td><td>30</td><td>5/27/2017 06:55</td><td>20</td><td>Nonmagnetic</td></tr><tr><td>16</td><td>R</td><td>136.4</td><td>143.9</td><td>7.5</td><td>1.15</td><td>1.34</td><td>15</td><td>5/27/2017 09:25</td><td>100</td><td>Nonmagnetic</td></tr><tr><td>17</td><td>R</td><td>143.9</td><td>148.6</td><td>4.7</td><td>1.30</td><td>1.59</td><td>28</td><td>5/27/2017 12:30</td><td>45</td><td>Nonmagnetic</td></tr><tr><td>18</td><td>R</td><td>148.6</td><td>152.6</td><td>4.0</td><td>0.59</td><td>0.90</td><td>15</td><td>5/27/2017 15:50</td><td>30</td><td>Nonmagnetic</td></tr><tr><td>19</td><td>R</td><td>152.6</td><td>158.3</td><td>5.7</td><td>0.88</td><td>1.28</td><td>15</td><td>5/27/2017 20:00</td><td>65</td><td>Nonmagnetic</td></tr><tr><td>20</td><td>R</td><td>158.3</td><td>163.3</td><td>5.0</td><td>0.90</td><td>0.80</td><td>18</td><td>5/27/2017 22:55</td><td>65</td><td>Nonmagnetic</td></tr><tr><td>21</td><td>R</td><td>163.3</td><td>165.5</td><td>2.2</td><td>0.45</td><td>0.40</td><td>20</td><td>5/28/2017 03:00</td><td>120</td><td>Nonmagnetic</td></tr><tr><td colspan="3"><strong>Hole U1504A totals:</strong></td><td>165.5</td><td>52.93</td><td>54.03</td><td>32</td><td></td><td></td><td></td><td></td></tr><tr><td colspan="10"><strong>368-U1504B</strong></td></tr><tr><td>1</td><td>1</td><td>0.0</td><td>88.2</td><td>88.2</td><td>4.17</td><td>4.17</td><td>43</td><td>6/4/2017 16:50</td><td>65</td><td>Nonmagnetic</td></tr><tr><td>2</td><td>R</td><td>88.2</td><td>97.9</td><td>9.7</td><td>0.10</td><td>0.16</td><td>1</td><td>6/4/2017 18:35</td><td>10</td><td>Nonmagnetic</td></tr><tr><td>3</td><td>R</td><td>97.9</td><td>107.6</td><td>9.7</td><td>0.18</td><td>0.31</td><td>2</td><td>6/4/2017 19:35</td><td>15</td><td>Nonmagnetic</td></tr><tr><td>4</td><td>R</td><td>107.6</td><td>117.4</td><td>9.8</td><td>1.65</td><td>2.13</td><td>35</td><td>6/4/2017 21:15</td><td>50</td><td>Nonmagnetic</td></tr><tr><td>5</td><td>R</td><td>117.4</td><td>122.1</td><td>4.7</td><td>0.69</td><td>0.89</td><td>14</td><td>6/5/2017 02:20</td><td>85</td><td>Nonmagnetic</td></tr><tr><td>6</td><td>R</td><td>122.1</td><td>127.1</td><td>5.0</td><td>1.30</td><td>1.72</td><td>28</td><td>6/5/2017 05:00</td><td>85</td><td>Nonmagnetic</td></tr><tr><td>7</td><td>R</td><td>127.1</td><td>131.8</td><td>4.7</td><td>0.90</td><td>1.23</td><td>18</td><td>6/5/2017 07:15</td><td>60</td><td>Nonmagnetic</td></tr><tr><td>8</td><td>R</td><td>131.8</td><td>136.8</td><td>5.0</td><td>0.55</td><td>0.55</td><td>9</td><td>6/5/2017 11:25</td><td>45</td><td>Nonmagnetic</td></tr><tr><td>9</td><td>R</td><td>136.8</td><td>141.5</td><td>4.7</td><td>0.85</td><td>1.6</td><td>16</td><td>6/5/2017 14:15</td><td>50</td><td>Nonmagnetic</td></tr><tr><td>10</td><td>R</td><td>141.5</td><td>146.5</td><td>5.0</td><td>0.84</td><td>0.84</td><td>13</td><td>6/5/2017 18:00</td><td>125</td><td>Nonmagnetic</td></tr><tr><td>11</td><td>R</td><td>146.5</td><td>151.2</td><td>4.7</td><td>0.92</td><td>1.6</td><td>16</td><td>6/5/2017 20:40</td><td>65</td><td>Nonmagnetic</td></tr><tr><td>12</td><td>R</td><td>151.2</td><td>156.2</td><td>5.0</td><td>0.95</td><td>0.92</td><td>20</td><td>6/5/2017 23:20</td><td>70</td><td>Nonmagnetic</td></tr><tr><td>13</td><td>R</td><td>156.2</td><td>161.0</td><td>4.8</td><td>0.30</td><td>0.43</td><td>6</td><td>6/6/2017 03:00</td><td>70</td><td>Nonmagnetic</td></tr><tr><td>14</td><td>R</td><td>161.0</td><td>166.0</td><td>5.0</td><td>0.75</td><td>0.85</td><td>49</td><td>6/6/2017 05:20</td><td>45</td><td>Nonmagnetic</td></tr><tr><td>15</td><td>R</td><td>166.0</td><td>169.8</td><td>3.8</td><td>1.85</td><td>2.25</td><td>49</td><td>6/6/2017 08:15</td><td>45</td><td>Nonmagnetic</td></tr><tr><td>16</td><td>R</td><td>169.8</td><td>175.8</td><td>6.0</td><td>0.75</td><td>0.84</td><td>13</td><td>6/6/2017 11:05</td><td>80</td><td>Nonmagnetic</td></tr><tr><td>17</td><td>R</td><td>175.8</td><td>180.5</td><td>4.7</td><td>0.75</td><td>0.92</td><td>30</td><td>6/6/2017 15:30</td><td>120</td><td>Nonmagnetic</td></tr><tr><td>18</td><td>R</td><td>180.5</td><td>185.5</td><td>5.0</td><td>1.50</td><td>1.84</td><td>30</td><td>6/6/2017 18:25</td><td>70</td><td>Nonmagnetic</td></tr><tr><td>19</td><td>R</td><td>185.5</td><td>195.3</td><td>9.8</td><td>1.85</td><td>2.65</td><td>19</td><td></td><td></td><td></td></tr><tr><td>20</td><td>R</td><td>195.3</td><td>200.0</td><td>4.7</td><td>0.79</td><td>0.98</td><td>17</td><td></td><td></td><td></td></tr><tr><td colspan="3"><strong>Hole U1504B totals:</strong></td><td>111.8</td><td>21.48</td><td>26.46</td><td>19</td><td></td><td></td><td></td><td></td></tr><tr><td colspan="3"><strong>Site U1504 totals:</strong></td><td>311.8</td><td>42.96</td><td>52.92</td><td>17</td><td></td><td></td><td></td><td></td></tr></tbody></table>

The center bit was recovered, and an RCB core barrel was dropped to begin coring. Sediments were recovered in Core 2R. The following two cores, 3R and 4R, contained limestone fragments. Basement rubble was recovered from Core 4R downhole. RCB cor-

ing continued through Core 20R at 200 m. Torque was erratic throughout coring operations in basement, and half cores were cut for all basement cores except Core 19R.
Coring was terminated due to time limitations after Core 20R, and the bit was recovered to the surface, clearing the seafloor at 0455 h and the rotary table at 1040 h on 7 June. Total time spent in Hole U1504B was 80 h (3.3 days). A total of 19 RCB cores were taken over a 111.8 m interval with 21.48 m of recovery (19%).

The rig floor was secured and made ready for the 946 nmi transit to Shanghai, China, by 1130 h on 7 June. The vessel was underway at 1200 h, ending Hole U1504B and Site U1504. After a 4 day transit, the *JOIDES Resolution* arrived in Shanghai, China, and Expedition 368 concluded with the first line ashore at Lingang Terminal at 1212 h on 11 June.

## Lithostratigraphy

We examined the lithostratigraphy of Holes U1504A and U1504B (Figure F6), which are approximately 200 m apart. Three lithostratigraphic units are defined; two sedimentary and one metamorphic. Lithostratigraphic Unit I is further divided into four subunits based on visual core description, smear slide and thin section inspection, and X-ray diffraction (XRD) analysis (see *Lithostratigraphy* in the Expedition 367/368 methods chapter [Sun et al., 2018]).

The recovered sediment succession extends from 17.50 to 134.80 m in Hole U1504A (Sections 3R-1 through 15R-1) and from 88.20 to 107.91 m in Hole U1504B (Sections 2R through 4R) and overlies greenish gray metamorphic rocks. The uppermost cores in Hole U1504A (Cores 1R and 2R) had no recovery.

Unit I is dominated by nannofossil and foraminiferal ooze with minor nannofossil-rich clay. Unit II is defined by a change from clay and ooze to bioclast-rich and clast-supported limestone with large benthic foraminifers. The contact between Units I and II was not recovered. Unit III is composed of fine- to medium-grained epidote-chlorite schist, calc-silicate schist, epidote-chlorite breccia, and marble.

## Unit description

### Unit I

Intervals: 368-U1504A-3R-1, 0 cm, to 12R-CC, 18 cm;
368-U1504B-2R-1, 0 cm, to 2R-CC, 17 cm

Depths: Hole U1504A = 17.50–112.66 m; Hole U1504B = 88.20–92.31 m

Age: Pleistocene to early Miocene

### Lithology and sedimentary structures

Unit I is dominated by nannofossil and foraminifer ooze with minor nannofossil-rich clay intervals at the top of the unit. The unit is divided into four subunits, IA–ID, based on changes in the biogenic component of the sediment and on color changes that coincide with subtle changes in the sediment components (Figure F7).

The boundary between Units I and II is marked by a change in lithology from ooze-dominated sediments to bioclast-rich and clast-supported limestone.

### Bulk mineralogy

The bulk mineralogy results of XRD analyses are listed in Table T2 and shown in Figure F8. Unit I sediments are mainly composed of calcite, quartz, plagioclase, K-feldspar, halite, pyrite, and clay minerals (including smectite, illite, kaolinite, and chlorite). Calcite is present throughout Unit I (mostly in the form of nannofossils and foraminifers) and becomes more abundant in Subunits IB–ID. Pyrite has a downhole trend similar to that of calcite. In contrast, the

percentages of terrigenous minerals (quartz, plagioclase, and chlorite) and authigenic halite have much lower contents in Subunits IB–ID than in Subunit IA. These general characteristics are consistent with the observed lithologic change from nannofossil-rich clay to foraminifer-rich nannofossil ooze.

### Subunit IA

Interval: 368-U1504A-3R-1, 0 cm, to 6R-1, 9 cm

Depth: 17.50–46.59 m

Age: Pleistocene

Subunit IA spans the recovered uppermost 20.09 m of Unit I and is dominated by nannofossil-rich clay, nannofossil ooze, nannofossil ooze with biogenic silica, and clay-rich nannofossil ooze (Figures F7, F9). The color of the ooze gradually changes from dark greenish gray to greenish gray and gray. The ooze comprises calcareous nannofossils and foraminifers; siliceous diatoms; sponge spicules; radiolarians; and terrigenous grains, including clay minerals, rare quartz, and feldspar (Figure F9). The sediment is well sorted and unlithified. The average grain size is clay, and silt is the maximum grain size.

Bioturbation is moderate to heavy. Individual burrows are difficult to distinguish but are mostly horizontal with recognizable *Planolites*, *Thalassinoides*, and *Cylindrichnus concentricus*. Distinct intervals (368-U1504A-3H-3, 0–34 cm, and 3H-4, 107–122 cm) contain more silt, and thin silt laminations are often disrupted by bioturbation (interval 3H-3, 69–56 cm). Layering is contorted at the base of the subunit.

The base of Subunit IA is marked by both a color change from greenish gray to pale brown and light brown sediment in Subunit IB and by a lithologic change that reflects an increase in the percentage of foraminifers. This increase is also seen in the progressive increase in measured carbonate content from an average of ~20 wt% in Subunit IA to an average of ~50 wt% in Subunit IB (Figure F32). Starting from approximately 1 m above this boundary, there is an uphole increase in biogenic silica, including diatoms, sponge spicules, and radiolarians. Below this boundary, Subunit IA is barren of siliceous microfossils.

### Subunit IB

Interval: 368-U1504A-6R-1, 9 cm, to 7R-3, 62 cm

Depth: 46.59–59.73 m

Age: middle Miocene

Subunit IB is 13.14 m thick and composed of light brown foraminifer-rich nannofossil ooze and pale brown nannofossil ooze with foraminifers (Figures F7, F9). The sediment is well sorted and unlithified. The average grain size is clay, and silt is the maximum grain size. Boundaries are horizontal or irregular. A contorted layer was observed in Section 368-U1504A-6R-4, 65–105 cm, possibly indicating intraformational slumping. Bioturbation is moderate to heavy.

The boundary between Subunits IB and IC is defined by a change from brownish foraminifer-rich nannofossil ooze (Subunit IB) to greenish nannofossil ooze with clay (Subunit IC).

### Subunit IC

Interval: 368-U1504A-8R-1, 0 cm, to 12R-1, 19 cm

Depth: 65.80–104.99 m

Age: middle–early Miocene