Hole 1243A is in 3882 m of water at 5°18.0541´N, 110°4.5798´W in the equatorial Pacific (Fig. F1). The site was chosen as the location for a future broadband seismic and complementary geophysical observatory by ION, the International Ocean Network, an Inter-association Committee of the International Union of Geodesy and Geophysics (IUGG). Hole 1243A was cased and cemented in place to accommodate future installation of the borehole seismometer.
Basement age at Hole 1243A is ~11 Ma, as established by spreading rate (van Andel et al., 1975) and by biostratigraphic and paleoceanographic analysis of the age of the near-basement part of the sediment column obtained from nearby Hole 851, drilled during Ocean Drilling Program (ODP) Leg 138 (Psias et al., 1995; Shackleton et al., 1995). This is a site with equivalent sediment thickness located ~0.5 nmi from Hole 1243A. Total seafloor penetration of 224 m was achieved, consisting of a sedimentary section of 121 m and 103 m of basement. Technical challenges in seating a 16-in casing unit into a hole with an 18-in bore led us to complete Hole 1243A by drilling with an 18-in rotary bit without coring, followed by insertion of a 10-in casing. The hole was cased to a depth of 212 m below seafloor (mbsf) and cemented in place, with the top of the cement at 199 mbsf. A schematic view of the cased legacy hole installed at Hole 1243A is shown in figure F9 of Shipboard Scientific Party, 2003. Casing with adequate bonding to basement rock was required because Hole 1243A is intended in the future to contain an observatory-quality broadband seismometer. The casing preserves the integrity of the hole for future reentry operations, whereas the cement bonding assures good mechanical coupling between the seismic sensors and the host rock. It is anticipated that the power, data storage, and telemetry infrastructure installed at and above the seafloor to support the seismic instrument will also be used for geomagnetic, geoelectric, and other complementary geophysical and oceanographic observatory sensors.
Technical complications related to installation of the casing were an impediment to logging and coring at this site. In order to achieve the goals identified for coring and logging of this site, an environment associated with lithospheric origin at a fast-spreading ridge crest, we tripped out of Hole 1243A and located a second, uncased hole (Hole 1243B) 600 m east of Hole 1243A, at 5°18.0543´N, 110°4.2544´W (Fig. F2). Given the limited time available to ODP Leg 203, and compounded by the requirement to curtail operations so the ship could make way to an alternative port of entry (Victoria, British Columbia, rather than San Francisco), we prioritized obtaining basement rock samples as the main objective of coring at that site, with wireline logging of the entire sediment and basement section as a secondary objective.
We jetted-in Hole 1243B to a point just above the sediment/basement interface and achieved a total penetration of 195 m, including 110 m of sediment and 85 m of basement. Sediment was recovered from the base of the sediment column in Hole 1243B in Core 203-1243B-1R (102–108 mbsf) comprising ooze of the same colors and lithologies as those recovered from Leg 138 Site 852 (Psias et al., 1995), which was located ~0.5 nmi south of Hole 1243B and which had nearly identical depth of sediment cover. The ooze is predominantly coccolithic, with small quantities of planktonic foraminifers, discoasters, radiolarians, iron oxide globules, and glass.
Basement samples were recovered from 17 cores (Cores 203-1243B-2R through 18R) obtained in the interval 108–195.3 mbsf. Recovery rates varied between 1.6% and 63.7% (average = 25%). In addition, the lowermost core, Core 203-1243B-19R, contained 5.3 m of drilling breccia/cuttings; however, postcruise analysis indicates that Core 19R comprised cuttings rather than breccias. The basalt samples from Hole 1243B largely consisted of mildly altered pillow basalts, including both aphyric and sparsely plagioclase and olivine phyric basalts. There was no evidence of thicker, massive basalt flows of the type encountered at ODP Site 1256, a basement section formed at ~15 Ma during an episode of superfast accretion on the East Pacific Rise (Wilson, Teagle, Acton, et al., 2003). Shipboard studies determined that the basement section comprised seven igneous lithologic units, one (Unit 2) of which comprised a single sample of limestone (Fig. F3). With the exception of lithologic Unit 4 (an alkali basalt), all igneous units were tholeiitic.
A reentry funnel was left in place in Hole 1243B to facilitate future installation of borehole instrumentation at a site identified by the Dynamics of Earth and Ocean Systems (DEOS) planning effort for geophysical observatory operations.
Wireline logging was carried out in Hole 1243B in the lower sediment section and throughout the basement section. Multiple trips were made with triple combination (triple combo) and Formation MicroScanner (FMS)-sonic tool strings, with a single trip of a Well Seismic Tool (WST) in a vertical seismic profile (VSP) configuration (Fig. F4). Cement bond and vertical inclination logs were also obtained from Hole 1243A, to confirm its suitability for installation of a broadband seismic sensor package.