04mm

Figure 63. Scanning electron micrograph of a deformed
micaceous chert grain from Guyet Formation conglomer-
ate. Note that the individual microcrystals of the chert
grain are elongate between the mica plates (sample of
which is noted by arrow) within the chert. Compare this
to the clean chert shown in Figure 64.

Some quartz grains were ruptured due to extension parallel
to the cleavage trace. Intervening spaces were filled with recrys-
talline fibrous and granular subgrains of quartz. The size of
the subgrains varies from 0.01 mm to 0.25 mm.

All quartz grains were measured. Grains that showed sub-
grain development were measured so as to include the subgrains.
Grains that have changed shape by solution and redeposition
upon the same grain will represent reasonable approximations
to geometric strain. Pressure solutions may transport material
from a grain or grains to deposit it in the pressure shadow of
another grain. This may cause the strain for that grain to be
overestimated, however, the area which donated the material
will be underestimated and a partial balance will occur.

Crushed quartz grains with their induced shape irregulari-
ties are not good geometric strain markers. The error intro-
duced, however, by including them in the measurements can
be no more than the errors associated with controlled grain
boundary sliding. Grain boundary sliding is suspected to have
occurred in the muddy conglomerate of the Guyet Formation.
Compaction by pressure solution shortening and general
dewatering of the clay matrix will have rotated quartz (and other
more viscous) grains towards the plane of flattening. This rota-
tion would be grain boundary sliding as opposed to the rota-
tion due to viscosity contrasts as outlined by Gay (1968a). Rota-
tions due to grain boundary sliding will alter the relationship
between the initial and final distribution, required as part of
the Elliott (1970) strain technique.

Chert. Although also composed of quartz, chert grains dis-
play a behaviour markedly different from grains of coarsely
crystalline quartz. Because of the small size of the quartz crystals

composing the chert grains it is not known whether the individ-

ual crystals exhibit undulatory extinction. Dislocation creep
therefore is hard to establish or refute with the aid of an opti-
cal microscope.

Chert grains show a flow-like behaviour, particularly where
quartz has impinged upon chert grains. The quartz grains
become partly enveloped by the chert. A difference between
chert and quartz grains is the high crystal boundary area of the
chert. To account for the behaviour of the chert it is postulated
that the high boundary area encourages diffusion flow. This
concept was discussed by Etheridge and Wilkie (1979) in rela-
tion to mylonites. Grain size reduction is argued to enhance
superplastic behaviour by increasing grain boundary area and
promoting diffusion flow and/or dislocation creep with depend-
ent grain boundary sliding (see also White, 1977).

Deformed chert (Fig. 62) from the Guyet Formation con-
glomerate has regular shaped individual crystals. Micaceous
chert (Fig. 63) from the same rock as Figure 62 contains individ-
ual crystals that are elongate. The sheet silicates of the mica-
ceous chert appear to have insulated the quartz crystals,
permitting them to deform individually.

The regularity of the quartz crystals in deformed pure chert
(Fig. 62) suggests that the individual crystals are continually
recrystallizing, either internally or by mass transfer between
grains. Material transported between grains is either deposited
as new grains in areas opened by grain boundary sliding or
becomes incorporated into the crystal structure of the pre-
existing grains, or both. Throughout the process, the individual
crystals did not become elongate as would be expected if they
were deforming by the grain boundary diffusion and flow
method outlined by Elliott (1973). His model involves shorten-
ing of grains parallel to the compression direction and elonga-
tion perpendicular to compression. Elongation takes place by
deposition of material removed from areas of the grain under
compression. This process may be operating in the micaceous
cherts, where individual quartz crystals are elongated preferen-
tially in the maximum extension direction.

Impure cherts with varying contents of mica exhibit stylo-
lite development under pinching conditions. The stylolites are
marked by concentrations of sheet silicates and minor fine
opaque matter.

Quartz grains that exhibit crushing phenomena at their edges
may be promoting similar behaviour as described for clean
cherts. The finer crushed fragments are forming more bound-
ary area and therefore, enhanced boundary diffusion. This argu-
ment suggests that within the area of a grain, both dislocation
and diffusion mechanisms can occur simultaneously. Only grain
size and possible impurity content would be the governing fac-
tor. As the quartz grain breaks down to form subgrains, diffu-
sion would increase as the grain boundary area increased. This
process may have operated in forming the chert-like accumu-
lation at the end of the large grain in Figure 64. Compare this
type of accumulation to that which fills pressure shadows, as
in Figure 65. Pressure shadow material is characteristically
elongate.

How does the diffusion process of the cherts affect its use
as a geometric strain marker? They would react very much as
theoretically required were it not for interference from grains
of different composition. Their originally angular to subrounded
shapes also do not accord well with theory, but that is less of
a problem than clast interference which induces new shape
changes.

97