Disharmonic Folds EarthCache
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Stop here, you will see complex disharmonic folds in this outcrop
of the (Cambrian) Chancellor Formation. Please pull into the
pullout on the southbound direction for a safe parking spot.
Entering the bushes is not required for this EarthCache, please
refrain from doing so to minimize the effects of an increased
number of visitors to this spot. Thanks!
The rock outcrop is part of the Chancellor Formation, which
consists of argillaceous limestone (light grey) interbedded with
comparatively thinner beds relatively pure limestone (dark layers).
The sediments which compose the Chancellor formation were deposited
during the Mddle to Late Cambrian period, which is about 500
million years ago.
The first thing you may notice is that the layering in the rock is
not horizontal, not even planar, but wavy. Not surprisingly, these
sedimentary rocks were not deposited this way. Sediments (and
sedimentary rocks) are generally deposited in planar, horizontal
layers. These sedimentary rocks, however, were twisted and
contorted by the heat and pressure involved in the mountain
building process. The heat reduces the strength of the rock while
the eastward-directed pressure shortened the rock in an east-west
direction. This phenomena is known as folding. When the folds are a
metre or less in size, geologist refer to these as small-scale
folds. The folds in this outcrop can be described as small-scale
disharmonic folds. Disharmonic folds are folds in which amplitude
decreases to zero within a few layers in the vertical direction or
die out within a few wavelengths in the horizontal direction. For
the musically-inclined (not me!), this can be likened to
disharmony.
As a result, the styles of folding between the light and dark
coloured beds are different. Look closely, the dark layers tend to
form broad, open, rounded folds. In contrast, you will notice that
the light coloured argillaceous layers form complex, tighter folds
that appear to mold to the folding in the dark coloured layers. The
style of folding is an indication of rock strength and rock
strength is often described in terms of competence. Rounded folded
layers are described as competent (stronger) layers, whereas the
layers that tend to mold to the shape of the competent layers to
fill the space in the hinges of folds are described as incompetent
(i.e. weaker in rock strength). The rapidly changing fold shapes
from layer to layer is what results in the disharmonic folding seen
here in this outcrop.
Locally, the dark limestone layers are broken and disconnected,
presumably due to local extensional stresses created during
folding. Extensional and shear fractures, some of which are filled
with calcite, are present within the competent dark limestone
layers, but not the argillaceous limestone. Fractures such as these
develope by snapping (i.e. brittle deformation), and were later
filled by the precipitation of calcite into the void created by the
breaking of this layer. This is in stark contrast to the
argillaceous layers that deform through comparatively ductile means
by being more malleable.
Structural geologists are able to use the information derived from
small-scale structures such as the ones seen here in this outcrop
to deduce the deformational style of the regional, mountain-scale
structure. The disharmonic folding style at this exposure provides
an approximation of the large-scale structural style of the Lower
Paleozoic rocks within the Porcupine Creek Fan Structure the
Western Main Ranges. This mega-scale (kilometre-scale) structure is
an anticlinorium of the southern Canadian Rocky Mountains.
To log this EarthCache, you must choose a fold
and perform the following tasks:
- Measure the vertical distance between a trough and a peak and
divide that number by two. This number is the amplitude (A) of the
fold. Email me the answer.
- Measure the horizontal distance between two similar positions on
neighbouring folds (ex. crest to crest or trough to trough). This
is the wavelength of the fold. Divide this number by two to get the
half-wavelength (M). Email me the answer.
- Determine the aspect ratio (P) of the fold by dividing A by M
(i.e. P = A/M). Email me the answer.
- The aspect ratio can be used to describe folds. With this number
(P), assign the fold you measured one of the following descriptive
terms:
Wide if P is between 0.1 and 0.25
Broad if P is between 0.25 and 0.5
Equant if P is between 0.5 and 2
Short if P is between 2 and 4
Tall if P is between 4 and 10
Email me which of these categories the fold you measured falls
into.
- Take a picture of your GPSr or yourself in front of the outcrop
and post it in your log.
A ruler or tape measure is recommended to perform
this work. Please do not log this cache until you are prepared to
complete all the required tasks.
Happy EarthCaching!
References
Fermor, P., Lynch, G. and Root, K., 2003. Canadian Society of
Petroleum Geologists Student Industry Fieldtrip, Southern Canadian
Rockies: May 9-12, 2003.
Additional Hints
(No hints available.)