IODP Expedition 304:
Ocean Core Complex Formation, Atlantis Massif
Site Summary: Site 1311
PDF file is available for download.
December 29, 2004
Hole U1311A: Latitude: 30° 10.609' N, Longitude: 42° 04.190' W, 2541 mbsl
Hole U1311B: Latitude: 30° 10.660' N, Longitude: 42° 04.217' W, 2505 mbsl
Site 1311 is located on a small knoll near the break in
slope where the lower, eastern flank of the corrugated dome meets the adjacent
volcanic, hanging wall block. Drilling at this site was directed toward
recovering a small amount of volcanic rock, fault rocks associated with the
presumed underlying detachment fault, and a shallow portion of the footwall.
Science Summary
Hole 1311A is located on the southern slope of the knoll. A
60 x 60 m survey with the VIT camera, documented a ~3600 m2 area of
mud-and rubble-covered seafloor, with a moderate slope to the south-southeast.
Along the northeastern corner of the survey area, a moderately to steeply SE
dipping, 20+ m high scarp oriented ENE (~075°) crops out, and is characterized
by rounded pillow structures. Scarp retreat has eroded any planar fault
surface; the lack of an unsedimented talus/debris fan at the base of the slope
suggests that the scarp may be relatively old.
One attempt to drill at this site penetrated 12 mbsf,
including 3.5 m of unconsolidated mud. Recovery from the hole produced 1.5 m
(13%) of fresh, vesicular, moderately plagioclase-olivine phyric basalt
pillows, with sparse glass preserved. Almost all pieces are angular and have broken along fracture surfaces
within pillows few, if any, were cut by the drill. The fracture surfaces are
discolored dark brown, and minimally altered. These observations suggest that
the core is derived from in situ pillows. Some piece interiors include
multiple, gray Liesegang bands indicating pervasive minor or incipient
alteration. Vesicles close to fracture surfaces are internally discolored brown
but not filled.
The basalt is dark gray to black in color, and characterized
by abundant (5-10%) seriate plagioclase that often occurs in radiating clumps.
Sparse euhedral plagioclase phenocrysts range upward in size to ~ 2 mm. In thin
sections from pillow interiors, the basalt appears fresh, with minimal
darkening of the matrix and occasional occurrences of orange to green
smectites, either in vesicles or in the vicinity of olivine. Randomly oriented,
acicular to prismatic plagioclase, ranging in size from <0.1 - 0.5 mm, makes
up ~ 40 % of the sample. Many of the plagioclase crystals have hollow or
swallowtail quench morphologies. Olivine microphenocrysts (~ 5%) occur either
in subophitic crystal clots with plagioclase or as euhedral microphenocrysts
ranging in size from 0.1-0.3 mm, and in shape from larger prisms to smaller
diamonds and more complex quenched forms. The remainder of the rock is the
devitrified glass matrix, dominated by plumose, quenched clinopyroxene with
anhedral interstitial plagioclase. Oxides are abundant, up to 2%, in the matrix
and commonly occur as complex quench morphologies, most likely ilmenite.
Vesicles occupy 3-5% of total volume. They occur in two forms: round (~0.1 mm)
and elongate, irregular, and commonly interconnected. Most are unfilled, but a
few are completely or partially filled by devitrified glass and a few,
especially near fracture surfaces, are filled by secondary green or orange
smectite. They generally range in size up to 0.5 mm, but in places are as large
as 2 mm (e.g., Section 304-1311A-1R-1 (Piece 6, 40-46 cm)). Where present,
glass is 1-3 mm thick with 50-100% palagonitization close to outer surfaces.
Small, but useful, volumes of fresh glass are preserved within the palagonite.
The one sample of pillow basalt (Sample U1311A-1R-1, 34-36
cm) analyzed for major and trace element geochemistry suggests that the basalt
is a primitive tholeiite, with a Mg# (100 molar Mg/Mg+Fe) of 66. The Site U1311
is geochemically similar to the basalt sampled in Hole 1310B and the
composition plots at the most depleted end of the field of Mid-Atlantic Ridge
volcanic glasses.
Operations Summary
Hole U1311A
A subsea camera survey revealed a smooth, sedimented
surface, with rare accumulations of angular to surrounded, submeter sized
boulders. Hole U1311A was initiated with an RCB BHA at 2040 hr 18 December
2004, in a region of our survey area devoid of visible rubble. High and erratic
torque were constant during the 25 hours required to drill Core U1311A-1R, and
rock falling into the hole had to be redrilled several times. Running the bit
back to bottom after recovering Core U1311A-1R, the driller determined there
was at least 3 m of fill in the 12 m hole. After several hours attempting to
clear the hole proved unsuccessful, Hole U1311A was abandoned.
Hole U1311B
After a second subsea camera survey starting ~100 m northwest of Hole U1311A, we
attempted to establish a hole by drilling without coring using a large diameter
bit. We hoped that the large diameter bit would be more efficient at clearing
the rubble falling into the hole, and we could begin coring with additional
weight on bit to allow faster and deeper penetration. Since drilling with any
rotary system was likely to encounter the same poor drilling conditions we
suffered with the RCB, we decided to attempt to drill the large diameter hole
with a wing-style reaming hammer bit. Even though we were not convinced the
hammer system with a wing-style bit could set casing in this environment, we
had seen it open a hole, and felt our attempt had a higher chance of success
than using a large diameter rotary bit. However, after 10 m of penetration,
forward motion with the bit ceased while we attempted to hammer through rubble
falling into the hole. After 19 hours, we were not able to advance the bit to the bottom of the hole, so the attempt was abandoned.
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