The volcanic basement was recovered from 41 holes at 33 sites along Transect EG65. The deepest recoveries were from Holes SEG22A (98 cm), SEG26A (110 cm), SEG27A (131 cm), SEG40A (118 cm), SEG60B (103 cm), and SEG61B (156 cm). The following observations and conclusions are drawn principally from igneous units, with additional information from basaltic clast units judged to sample local volcanic basement. Two distinctly different basalt series occur at Transect EG65, as detailed below.
The cored igneous units are dominantly massive, fine-grained, moderately to highly phyric basalts. Igneous units from Transect EG65 have diverse phenocryst assemblages composed of plagioclase as the volumetrically most abundant phenocryst and with or without minor amounts of clinopyroxene and olivine. Porphyritic to glomeroporphyritic (Fig. F11A) and seriate textures are common. Glomerophyric intergrowths occur with all three phenocrysts phases but are most common in the highly plagioclase phyric lavas. Indications for simultaneous growth are commonly seen in the form of "bow-tie" intergrowth between plagioclase and clinopyroxene (Fig. F11A). Plagioclase often retains a euhedral, lath-shaped fabric, whereas altered olivine often appears with subrounded to euhedral shapes (Fig. F11B). The groundmass is typically intergranular and intersertal and composed of plagioclase, clinopyroxene, opaque minerals (mostly Fe-Ti oxides and minor amounts of sulfides), and mesostasis. Slowly cooled, fine- to medium-grained groundmass interiors of flows and clasts show subophitic intergrowth between plagioclase laths and interstitial to granular clinopyroxene. Such flow interiors often retain a relatively fresh appearance (Fig. F11C). A weak trachytic groundmass texture defined by plagioclase laths is seen in many upper series basalts. Sparsely plagioclase phyric basalts are also common. These lavas often have large vesicles commonly filled with gray clay and zeolite (Fig. F11D).
A less common suite of lavas contains olivine and plagioclase phenocrysts with minor amounts of clinopyroxene. These lavas are highly to moderately phyric and have a distinctive red coloration resulting from the alteration of olivine to iddingsite and lizardite (Fig. F12A). Chromium spinel occurs as inclusions in olivine phenocrysts (Fig. F12B) and in the groundmass of the most olivine-rich samples. Modal proportions of plagioclase and olivine phenocrysts are highly variable (Figs. F12C, F12D, F13). Small (<1 cm) xenoliths composed of olivine and green pyroxene (chromium diopside?) are reported in Unit I-1 from Holes SEG51B, SEG55B, and SEG55C.
The occurrences of these two petrographically distinct suites of lavas (highly to moderately plagioclase phyric and highly to moderately olivine phyric), as illustrated by rock modes in Figure F13, form the basis for dividing the drilled lava succession into a lower series and an upper series. Sites landward of Site SEG51 are dominated by highly to moderately olivine-plagioclase phyric basalt, whereas basalt recovered seaward of Site SEG45 are typically aphyric to plagioclase-clinopyroxene phyric basalt.
Although most of the igneous units are described as massive, some show diffuse vesicle or amygdule bands (2–3 cm thick) roughly perpendicular to the core axis. Occasionally vesicles are flattened parallel to the banding (Holes SEG27A, SEG29A, SEG40A, and SEG44A). Rounded to highly irregularly shaped vesicles (0.5–4 mm in diameter) are present in most of the units. The vesicles may be elongated in bands or pipes (Fig. F6) and are either void or filled with secondary minerals (Fig. F11). Quartz, calcite, zeolite, and clay typically fill vesicles, vugs, and fractures. Rusty-brown weathered surfaces and fractures found on fragments in Units I-1 from Holes SEG52A and SEG63A may represent oxidized flow tops. Likewise, the highly fractured and crumbled basalt found at the top of Holes SEG45A and SEG51A resembles flow top rubble, whereas conjugate fracture sets and weakly developed slickensides described from Hole SEG21A suggest some postemplacement tectonic movement.
The contact between the sediment cover and the underlying basaltic basement is not preserved in any of the holes drilled from Transect EG65. The surface of the volcanic basement was preserved in two holes. Glacial striations are found on the top surface of core from Hole SEG40A. Remarkably, live corals attached to the glacially polished top surface were recovered from Hole SEG27A (see Fig. F5). Coral and other organisms attached to or coating larger pebbles and drilled basalt fragments and boulders from Hole SEG21A shows that certain portions of the seabed were void of fine-grained sediment covers.
It is also noteworthy that melanocratic gneiss was recovered in Hole SEG32A. Thin section examination shows an assemblage of hornblende, biotite, quartz, magnetite, ilmenite, and accessory zircon, apatite, sphene, and chalcopyrite (Fig. F14). Although no radiometric age information is available, the mineralogy and gneissic fabric of this cored basement are similar to the immediately adjacent onshore Precambrian crust.