Site U1309 (Fig. F2) is located on the central dome of Atlantis Massif, 15 km west of the median valley axis of the MAR, where the seafloor is interpreted to coincide with a gently sloping, corrugated detachment fault surface. Two drill holes at this site (Holes U1309B and U1309D) penetrate a multiply intruded and faulted crustal section, providing core that documents the interplay between magmatism and deformation prior to, during, and subsequent to a period of footwall displacement and denudation associated with detachment faulting. Five shallow-penetration holes (Holes U1309A and U1309E–U1309H) (Fig. F2) were drilled during Expedition 304 to sample the sedimentary carapace and upper few meters of the basement, test the hypothesis that the upper surface coincides with the detachment fault, and help constrain the temporal history of denudation (see Shipboard Scientific Party, 2005). In this report, we focus on results from Hole U1309D, which is where all Expedition 305 coring and logging operations were carried out.
Site selection was based on a combination of geological and geophysical data, balancing the details of seafloor character with larger-scale objectives. Centered within the gently sloping, corrugated, and striated dome (Cann et al., 1997), Site U1309 is within a positive Bouguer gravity anomaly and close to the area where high seismic velocity is inferred to indicate the presence of unaltered ultramafic rocks within several hundred meters of the seafloor (Blackman et al., 1998; Collins et al., 2001). Argo II imagery and Alvin dive mapping previously showed that the seafloor is covered by a thin layer of unconsolidated sediment, deposited on bedrock and in places interrupted by lineated rubble fields (Blackman et al., 2004). In areas without significant loose sedimentary cover, a thin cover of lithified carbonate caps the underlying low-relief basement. Dredge and Alvin sampling indicate that loose, angular fragments on the central dome include low-grade metabasalt and serpentinite (Blackman et al., 1998, 2004).
Most of the holes composing Site U1309 are located in an area with 2–4 m of unconsolidated sedimentary deposits above basement. During Expedition 304, a 60 x 50 m survey with the vibration-isolated television camera on the drill string documented an ~2000 m2 area free of cobble- to boulder-sized rubble. The area is ~280 m south of an Argo II track (run 039) and an Alvin dive (3642), both from cruise AT3-60 (Blackman et al., 2004), and west of EW0102 MCS Line Meg-4, at common midpoint 4100 (Canales et al., 2004). Towed ocean-bottom instrument and DSL120 side-scan sonar data show spreading-parallel striations crossing this area. A gentle northeast slope coincides with the southern flank of the corrugation the site penetrates. Drilling in Hole U1309D took place over two periods for a total of 15 days during Expedition 304; penetration reached 401.3 mbsf. The hole was spudded using a hammer drill with 133/8 inch casing, in an effort to provide stable reentry for a deep hole. No rock was recovered in the upper 20.5 m of the hole. Below 20.5 mbsf, coring was accomplished using a rotary core barrel bit. Despite rough sea conditions during some of the drilling, recovery rates were very good—64% overall. As drilling conditions in the pilot hole (Hole U1309B, Expedition 304) were very good, casing below 20 m in Hole U1309D was deemed unnecessary.