Geology Reference
In-Depth Information
1200
378
1250
Upper
fracture
interval
1300
402
1350
423
1400
Middle
fracture
interval
1450
450
Fig. 5.
Fracture plane angle versus
depth. The skeletal 'interruptions'
exhibit a wide scatter in inclination,
but most are in excess of 30º. Note that
relatively few fractures occur in the
hemipelagic intervals between the frac-
ture intervals.
1500
459
Lower
fracture
interval
468
1550
0
10
20
30
40
50
60
70
80
90
Fracture angle (from horizontal)
combination of the two, as Melim
et al
. (2001)
suggest a temporal overlap of celestite precip-
itation and aragonite neomorphism. They also
argue that abundant celestite precipitation would
require the import of sulphate by active fl uid cir-
culation. Even with the temporal uncertainty, it is
interesting to note that the celestite age is coinci-
dent with the end of a major progradational pulse
in the latest Pliocene (see Discussion below).
DISCUSSION OF FRACTURE ORIGIN
Origin of the fractures: a loading model
The origin of the fractures is clearly not related
to drilling (as celestite partly infi lls many
fractures), and there is no evidence of local tec-
tonic forces that could generate such localized
fractures. Several key features and lines of evid-
ence (shear/cataclastic fracture type, steep angle,
conjugate fracture sets orientated perpendicular
to the margin, and downward cumulative fracture
length) suggest that fracture formation is related to
vertical compression from sediment loading. The
proposed model for fracture formation involves
several factors: the integration of the coarse, per-
meable skeletal sand 'interruptions', a prolonged
period of non-deposition or erosion associated
with a sequence boundary, and the subsequent
rapid deposition of overlying sediment related
to clinothem progradation
(
Fig. 9
)
. The fractures
occur within a 45-m-thick interval immediately
Fracture orientation
The dip azimuth of 19 through-going shear frac-
tures, determined using magnetic declination,
shows a bi-directional, conjugate distribution
(Fig. 8). Two general groupings are approximately
180° apart, one westward and one eastward.
These two orientations are roughly equivalent to
seaward (west) and bankward (east) of the bank
margin (oriented just east of north). The majority
of the fracture planes dip to the west (14 of 19) in
the direction of the depositional slope and basin.
