Geology Reference
In-Depth Information
METHODS
Prof. Swart and provided for this study. This
data point followed the methodology described
by Swart
et al
. (2001) for the strontium-isotope
stratigraphy of core Clino material. The celestite
value was normalized to a NIST-987 standard of
0.710248 and the age determined by the look-up
table of McArthur
et al
. (2001), which is calibrated
to the Berggren
et al
. (1995) timescale.
Core data and fracture characterization
The high core recovery (>90%) in the middle and
lower parts of core Clino provide a nearly continu-
ous record of early diagenetic features such as the
fractures reported here. One-third of the whole
core (6.3 cm diameter) was cut and separated for
archive purposes. The remaining two-thirds of the
core was used to measure and to collect samples.
The depth values reported here are consistent with
those in Ginsburg (2001), where the depths are in
metres below mud pit (subtract 7.3 m to obtain
depth below mean sea level). The depth interval
from about 375 to 450 m contains distinct fractures
that were fi rst noted on site during drilling. The
fractures in the slabbed core were characterized
by measuring fracture plane inclination, fracture
length, fracture aperture, and noting if the fracture
was open or fi lled. The vertical extent of the frac-
ture was also recorded and was used to provide a
rough estimate of total fracture length per metre
of core. This calculation is used to approximate
fracture density.
Uniaxial deformation experiments
To obtain an estimate of the force needed to frac-
ture rock of similar composition to that which
hosts the fractures, two uniaxial compression
tests were conducted. Two whole-core samples
from the fractured zone (440 and 447 m) were
subjected to compression tests at ETH-Zurich.
The effective pressure of brittle failure was deter-
mined with standard methods of comparing the
confi ning pressure minus the pore pressure (water
at 10 MPa). The nature of the fracture and fault
produced was also recorded.
RESULTS
Fracture orientation
Fracture data set
The dip azimuth of the fracture plane was calcu-
lated using magnetic declination. This procedure
consisted of measuring the fracture dip orienta-
tion relative to an arbitrary red line marked on the
core (used to confi rm the up direction). The offset
angle of the red line was then measured relative
to a palaeomagnetic plug drilled from that piece
of core. The declination direction measured from
the plug (see McNeill
et al
., 2001) was then used to
reorient the core piece (red line), and calculate the
true dip azimuth of the fracture plane. This reori-
entation method is fairly robust given the strength
and stability of the magnetic signal in this section of
the core (McNeill 1997, McNeill
et al
., 2001). This
method does, however, have a certain amount of
inherent error (such as magnetometer error, least-
squares calculation of declination, preservation of
true declination), and the error range is
estimated
at
The fractures in core Clino were grouped into
three classes: through-going shear, axial split, or
cataclastic shear (following the usage of Paterson,
1978). The through-going shear type was often
a simple, single fracture or fault with relatively
little effect on the adjacent host sediment
(Fig. 3a and b). The axial fractures are those ori-
ented vertically or near vertically (>70°) along
the axis of the core (Fig. 3c). This type often had
multiple fractures that radiated outward from a
slightly larger central fracture, although it was dif-
fi cult to trace these subfractures in a 6.3 cm wide
core. The cataclastic shear fracture type showed
a zone, 10-20 cm long, that contained abundant
fractures of differing orientations, lengths and
aperture sizes (Fig. 3d and e). The cataclastic
zones sometimes had a central through-going
fracture surrounded by subfractures, or termin-
ated abruptly on a bed of differing composition
and integrity.
The inclination of the fractures (
n
= 151) showed
a skewed distribution, with most fractures biased
to higher angles between 30° and vertical (Fig. 4).
The mean inclination was ~57°, with a large
standard deviation (~24°). The fracture inclina-
tion with depth in each of the intervals shows a
5°. All of the orientation data are from pieces
of core that contain either a magnetic plug sample
or are from adjacent pieces that could be fi tted
together with a plug-oriented core piece.
Sr-isotope ratio on fracture fi ll
A single strontium-isotope ratio on the mineral-
ized fi ll material (celestite) was determined by
