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doi:10.2204/iodp.proc.304305.103.2006

Shallow-penetration cores

Among our scientific objectives at the central dome of Atlantis Massif was capturing the sediment cover draping the top of the dome and recovering rock from the uppermost part of the basement directly below this cover. We reasoned that the denudation history of the footwall via detachment faulting might be constrained by paleoceanographic indicators within sedimentary deposits, if they could provide limits on age of exposure of this surface at the seafloor. In addition, the lithified carbonate cap previously observed and sampled in submersible dives was known to contain microfossils. If the exposure of the western domal surface occurred significantly earlier than exposure at the eastern edge (the termination at the hanging wall block), sampling along a spreading-parallel transect across the top of the dome might document the denudation history of the dome. In addition, the uppermost basement could preserve the fault rock formed while the domal surface was exposed during unroofing along the detachment.

Although rotary coring was an effective approach to recovering basement at Atlantis Massif, in order to keep the rotary bit in the hole while recovering the first cored interval, it is commonly necessary to core to >10 mbsf with the first core barrel. As a result of pipe flexure and slow penetration rates in this initial coring interval, recovery in hard rock is generally poor (commonly <20%). In addition, circulation of drilling fluid, used to keep the bit clear of impacted sediment, tends to flush away thin surface sediments during RCB coring.

During operations at Site U1309, we tested several methods to improve recovery from the sediment cover and uppermost basement. These included pushing the RCB BHA into the sediment without rotation or circulation (Hole U1309A), routine RCB coring with rotation and circulation to sufficient depth to ensure the pipe did not pull out of the hole after the first cored interval (Hole U1309B), shallow penetration (1–2 m after contact with hard formation) with rotation but no circulation (Holes U1309E and U1309F), shallow XCB coring with rotation and no circulation (Hole U1309G), and APC piston coring (same location as Holes U1309G and U1309H). The core barrel separated during the APC coring attempt, and no core was recovered. The other coring attempts recovered intervals of drilling disturbed, unlithified, unconsolidated sediment and small fragments of basement from beneath the sediment cover. Brief descriptions of these cores are summarized below.

Core 304-U1309A-1R

Core 304-U1309A-1R (0.0–2.0 mbsf) was a push core without rotation or circulation; 1.77 m of drilling-disturbed calcareous microfossil ooze was recovered. The upper 25 cm of Section 304-U1309A-1R-1 is normally graded drilling slurry, with subcentimeter chips of basalt and serpentinite with mineral grains near the base of the interval. From 25 cm to the base, Section 304-U1309A-1R-2 consists of tacky, light tan calcareous ooze with abundant microfossils. Section 304-U1309A-1R-CC is completely disturbed, but also contains subcentimeter chips of basalt and serpentinite.

Core 304-U1309B-1R

The first core from Hole U1309B was cut with rotation and circulation to 15.5 mbsf. A total of 2 m of drilling-disturbed calcareous microfossil ooze was recovered above several cobbles of basalt (see “Igneous petrology”). The upper 120 cm of sediment is normally graded drilling slurry, with millimeter-sized chips of basalt, serpentinite, and, possibly, carbonate. From 120 to 135 cm is light tan, soupy sediment. To 10 cm in Section 304-U1309B-1R-3 is light tan ooze with abundant microfossils.

Core 304-U1309E-1R

Core 304-U1309E-1R was cored 10 m east of Hole U1309D to 1 mbsf with rotation but no circulation. A total of 5.6 m of soupy to tacky, severely drilling-disturbed, light tan calcareous microfossil ooze was recovered. Two intervals in Section 304-U1309E-1R-1 (0–8 and 20–28 cm) are stiff, silty clayey fine sand. Section 304-U1309E-1R-2 is soupy mud, with a thin interval at 8–13 cm of stiff, silty sand. At 93 cm are thin laminae of green, sandy silt. At 6–20 cm in Section 304-U1309E-1R-3 is a smear of green silty, clayey sand with rare subcentimeter fragments of greenschist-facies-altered basalt. Overall, Section 304-U1309E-1R-3 is soupy and has a slightly greener cast than sections above with disseminated black sand. The interval from 0 to 6 cm in Section 304-U1309E-1R-4 is similar to the green silty sand in Section 1R-3. The rest of the section is similar to the soupy to tacky, severely disturbed mud from Section 304-U1309E-1R-3. The upper part of the core catcher contained stiff clayey silt with abundant foraminifers and fine sand-sized black particles. The bottom of the section contains a 7 cm rounded fragment of highly altered metabasalt with tremolite and chlorite. Ghosts of stretched fragments are discernible in hand specimen. One edge of the basalt is in contact with foliated tremolite schist with rare chlorite.

Core 304-U1309F-1R

Core 304-U1309F-1R was cored with rotation but no circulation to 5.6 mbsf (1 m into hard formation); 6.09 m of light tan, soupy, severely disturbed calcareous ooze was recovered. At 45 cm in Section 304-U1309F-1R-2 is a small interval of semi-indurated sandy clay.

Two thin intervals (58–62 and 67–72 cm) in Section 304-U1309F-1R-4 are coarser grained, semi-indurated sandy mud that crushes easily with light pressure. At 60 cm in Section 304-U1309F-1R-4 are several centimeter-sized fragments of Mn-encrusted rock. On one side of the largest piece are radiating fans of minerals that may have been amphibole. Below 4 cm in Section 304-U1309F-1R-6 is a distinct color change to darker tan, and the sediment has a denser texture (which may have been imparted by packing mud in the core liner with a plunger). A centimeter-sized fragment of brown-green, pervasively altered basalt is in the top of this section. The lower part of the core catcher contains abundant fragments of Mn-encrusted rock and several pieces of pervasively altered, brown-green basalt. Some possible amygdules are present, and a few blades of amphibole are preserved along a fracture.

Core 304-U1309G-1X

Hole U1309G is located within a few meters of a submersible-deployed marker where lithified carbonate was observed to drape basement. Core 304-U1309G-1X was cut with the XCB and cored to 3.5 mbsf without circulation. The recovered core is dark tan silty clay with abundant microfossils, dark sand-sized rock fragments, and mineral grains. Rare disturbed streaks of gray-green clay and rare millimeter-sized green metabasalt fragments are also present. At 38–40 cm in Section 304-U1309G-1X-1 is a 3 cm clast of indurated ooze and red-brown hyaloclastite. Two 2–3 cm thick intervals of carbonate-free hyaloclastite bracket a 4 cm thick interval of light tan calcareous ooze with abundant subcentimeter sized fragments of gray-green metabasalt. A thin (<0.5 cm) layer of calcareous ooze with sharp, subhorizontal contacts separates the lower hyaloclastite layer from a 3 cm thick layer of densely packed gray-green silty clay. Below the clay interval is a clayey silt matrix-supported conglomerate of rounded, mostly metabasalt fragments. This material was packed into the XCB shoe and appears to be drilling-reworked sedimentary deposit.

Core 304-U1309H-1R

Hole U1309H was cored at the same location as Hole U1309G, based on GPS coordinates. A single RCB core was recovered (Core 304-U1309H-1R; 0.0–4.0 mbsf).

The upper few centimeters of this core includes several 1–3 cm sized rounded pieces of hyaloclastite and Mn-encrusted, semilithified microfossil-bearing ooze. The pieces contain submillimeter-sized basalt fragments and fresh and palagonitized basaltic glass. Beneath these fragments in the core catcher is a rounded piece of talc-tremolite schist.

This schist has a strong foliation with apparent folds, suggesting that it formed during ductile or semibrittle deformation. The schist is overprinted by a brecciation event, resulting in minor disruption of the foliation. The lowermost part of the core recovered from Hole U1309H is a highly altered brecciated basalt or diabase. The brecciation is localized, separating regions that are relatively undeformed. This texture suggests that the rock did not experience significant shear displacement, although it may represent part of the brittle process zone adjacent to a fault.

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