FOOTWALL SITE

Site U1309 is located on the central dome of Atlantis Massif, 15 km west of the median valley axis of the MAR, where the seafloor coincides with what is interpreted to be 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) were designed to sample the sedimentary carapace and upper few meters of the basement, to test the hypothesis that the upper surface coincides with the detachment fault, and to help constrain the temporal history of denudation. Collected sedimentary deposits may provide constraints on the timing of exposure across the dome based on the age and isotopic character of preserved microfossils. Basement rock sampled in these short holes provides initial information on deformation and alteration within the exposed fault and, perhaps, rock adjacent to the fault zone.

Site selection was based on a combination of geological and geophysical data, balancing the details of seafloor character with larger-scale objectives attainable if deep penetration of the footwall is successful. Centered within the gently sloping, morphologically corrugated, striated dome (Cann et al., 1997), the site coincides with gravity anomaly and seismic velocity maxima that indicate unaltered ultramafic rocks are likely to be present within several hundred meters below 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).

Site U1309 comprises eight holes drilled within 2 km of one another and along a spreading-parallel corridor (Fig. F3). The first five holes, Holes U1309A–U1309E, are located within 30 m of each other, in an area with 2–4 m of unconsolidated sedimentary deposits above basement. A 60 m x 50 m survey with the vibration-isolated television (VIT) camera on the drill string documented a ~2000 m² area where both a single-bit pilot hole and a deep penetration hole could be initiated within an 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), just south of NOBEL line 10 and west of EW0102 MCS Line Meg-4, at common midpoint 4100 (Canales et al., 2004). Towed ocean-bottom instrument (TOBI) 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. Principal geologic results from the pilot and deep penetration holes are presented in subsequent sections of this summary.

The series of shallow penetration holes included in this footwall site are located adjacent to, 280 m northwest of, and 1.6 km northeast of the deep hole (Hole U1309D). The motivation for this series of holes is two-fold: first, to check for possible fossils or isotopic signatures in the sedimentary deposits to constrain the exposure age of the hypothesized detachment fault and, second, to attempt recovery of possible fault rock at the top of the domal surface. The first shallow penetration core was obtained in Hole U1309A. Overcoring in the top interval of the deep penetration holes (U1309B and U1309D) precluded meaningful recovery from the sedimentary deposits and several meters of basement immediately below. Additional shallow penetration holes were drilled in an effort to achieve that goal (Holes U1309E–U1309H).

Before initiating coring in Hole U1309A (30°10.1081'N, 42°07.1101'W; 1642 mbsl), we obtained a temperature measurement and seawater sample for microbiology and geochemistry. Bottom water at this site has temperature = 5.33° alkalinity = 2.16 mM, and salinity = 35.5 g/kg. These values are typical for the water depth and North Central Atlantic location of the site. A push core in Hole U1309A (Table T1) recovered ~2.5 m of unlithified tacky mud above bedrock. This microfossil ooze (with foraminifers and pteropods) includes mineral grains (fresh olivine and pyroxene), as well as fish remains. A sample of sediment for microbiological study was collected from Section 304-U1309A-1R-2, 48–60 cm, ~20 cm above basement.

In an effort to assess drilling conditions and begin geologic characterization of the expected detachment fault zone, a single-bit pilot hole was drilled at Site U1309. Hole U1309B, cored at the same location as Hole U1309A, was cored to 101.8 mbsf. Recovery was good (overall average = 46%), increasing significantly (to an average of 52% for 30–100 mbsf) below the upper, very slow-drilling 25 m. The hole deviates from vertical by 7° toward the northeast, and this was probably a factor in the reduced quality for one of the downhole logging measurements.

The attempt to set casing for the hard rock reentry system was not successful in Hole U1309C (30°10.1081'N, 42°7.1209'W; 1638 mbsl). The hole was abandoned with ~25 m of 13³/₈ inch casing pipe standing above the seafloor; it is 20 m east of Hole U1309B and ~30 m southwest of Hole U1309D.

Drilling in Hole U1309D (30°10.1195'N, 42°07.1131'W; 1645 mbsl) 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 (RCB) bit. Despite rough sea conditions that were experienced during some of the drilling, recovery rates were very good—58% overall. The section from 108 to 126 mbsf (Cores 304-U1309D-18R to 21R) had very low recovery, including one empty core barrel, and a second with only 14% recovery. Logging data suggest this low recovery zone may coincide with a fault zone. Aside from the low recovery interval and the upper 30 m, recovery rates averaged 68%. As drilling conditions in the pilot hole (Hole U1309B) were very good, casing below 20 m in Hole U1309D was deemed unnecessary. This gave us time to core the upper ~100 m during our initial occupation of the hole, providing an opportunity to assess cross-hole correlation of lithologic units and structure. The second period of coring followed drilling at Sites U1310 and U1311 and deepened the hole to 401.3 mbsf.

Hole U1309E (30°10.1207'N, 42°07.1057'W; 1645 mbsl) was offset 10 m east of Hole U1309D for an attempt to recover the sediment and upper meter of basement using the RCB. Disrupted sediments were obtained, as were several fragments of metabasalt.

The second attempt at shallow penetration was made in Hole U1309F (30°10.1999'N, 42°07.2518'W, 1645 mbsl) ~280 m to the northwest of Hole U1309D in an area where unconsolidated sedimentary deposits are less widespread than at the sites of Holes U1309A–U1309E, and lithified carbonate cap rock was mapped with the Alvin and Argo during cruise AT3-60 in 2000. A brief camera survey confirmed the basic setting, although some loose sediment occurred within a few meters of the first contact of the drill bit at the seafloor. Despite clear indications that we drilled over a meter into hard rock, recovery included only disrupted sediment and a few fragments of metabasalt. No chips of lithified carbonate were recognized. We discontinued RCB shallow penetration attempts and switched to the extended core barrel (XCB) bit for the next attempt.

Hole U1309G (30°10.5379'N, 42°06.3179'W; 1872 mbsl) was sited 1.6 km northeast of Hole U1309D, in an area characterized by a broad region of variably lithified carbonate deposits above basement. A brief camera survey confirmed this assessment, and the hole was located within site of a marker left by the Alvin in 2000. The hole was spud into stepped and platy, lithified carbonate sediment. Coring to 3.5 m using an XCB bit recovered 0.91 m of microfossil ooze, with three thin (2–3 cm) interlayers of basaltic hyaloclastite. Glass from the hyaloclastite is oxidized palagonite. No lithified carbonate or intact basement rock was recovered. The sequence of fossiliferous ooze, hyaloclastite, and a clayey material with rounded, largely metabasalt clasts may provide useful post-exposure data. The latter could be a sedimentary conglomerate, but we cannot rule out significant reworking due to drilling in this lowermost interval.

A second attempt at basement recovery from a shallow penetration hole at the same location as Hole U1309G was possible when logging activities in Hole U1309D were stopped early due to tool hang-ups apparently associated with falling rock. Rather than risk either the tools or hole, the remaining time on site was used to RCB core a few meters at essentially the same location as Hole U1309G. This eliminated the need for camera survey. Drilling for 4 h penetrated to 4 mbsf; recovery from this hole was 0.19 m and includes pieces of basalt and talc-tremolite schist along with one piece of diabase cataclasite (Fig. F4). Despite the small return, these samples are significant. The talc-tremolite schist is similar to fault rocks recovered near the top of the southern ridge at Atlantis Massif (Schroeder and John, 2004) and at 15°45'N on the MAR (Escartin et al., 2003). Fracture intensity in the diabase is minor, suggesting fairly low strain, but consistent with the sample being part of a process zone associated with a fault system. Although these samples are minimal, they provide the first direct evidence that the corrugated central dome of Atlantis Massif is an exposed detachment. An Alvin sample within this spreading-parallel corridor showed similar talc rock (Blackman et al., 2004), and there were a few chips of talc-tremolite schist in the top core from Hole U1309B, but neither of these samples could be proven to be in place. Prior confirmation of the existence of a fault zone capping the dome had been available on the southern ridge where microstructural analysis of dive samples (Schroeder and John, 2004) showed a 50–100 m thick zone where brittle deformation was concentrated just below the top of the south face of the massif.

A total of 12 samples for microbiological investigations were taken from core recovered from Holes U1309B and U1309D. Sample depths ranged from 0.45 to 396.5 mbsf, and all major rock types were included: carbonate sediment, basalt, diabase, gabbro, and serpentinized peridotite. Onboard cultivation studies indicate growth of matter from two altered gabbro samples at elevated temperature, based on positive fluorescence tests. Onshore analyses are required confirm that this is a microbial signature as opposed to being due to inorganic material.