We drilled a cased reentry hole (Hole 1243A) at 5°18.0541'N, 110°4.5798'W in the eastern equatorial Pacific, the location of a future Dynamics of Earth and Ocean Systems (DEOS) multidisciplinary observatory. The drill site was located in 10- to 12-Ma lithosphere at a water depth of 3882 m. The hole was drilled to a total depth of 224 m, which included 121 m of sediment and 103 m of basement penetration. We inserted casing to 212 meters below seafloor (mbsf) and cemented the casing in place, with the top of the cement at a depth of 199 mbsf. Subsequent logging showed that the casing was well bonded to basement in the lower 40 m and that the deviation of the hole never exceeded 1° from vertical. Casing complications obviated coring and scientific logging this hole. Hole 1243A will subsequently be used to install an observatory-quality broadband three-component seismometer (0.0015 Hz) as well as a high-frequency three-component seismometer (120 Hz) to ensure high-fidelity recording over the range of frequencies normally recorded by the terrestrial Global Seismic Network. The seismic system, as well as other instrumentation associated with the observatory, will be connected to a DEOS mooring for both power and high-speed data telemetry to a land station and the Internet.
The equatorial site satisfies two scientific objectives of crustal drilling: (1) it is located in one of the high-priority regions for the Ocean Seismic Network and DEOS, and (2) it is in oceanic crust created by fast seafloor spreading, providing a rare opportunity to examine crustal genesis, evolution, and crust/mantle interaction for a seafloor-spreading end-member responsible for generating the majority of the oceanic lithosphere. To satisfy the secondary objective, we drilled a second uncased hole (5°18.0543'N, 110°4.2544'W) fitted with a reentry funnel 600 m east of Hole 1243A. Rotary coring alone was used in an effort to sample the sediment/basement interface as well as the uppermost fast-spreading lithosphere. Hole 1243B is characterized by 110 m of sediment and a total penetration of 195 m. Core recovery throughout basement averaged 25%, and the lower sediment and basement were logged. This is a multidisciplinary project that primarily represents the interests of the National Science Foundation's component of the international DEOS planning effort and the International Ocean Network.
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