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Grabens
The grabens in the Needles District
of Canyonlands National Park are
a system of linear collapsed valleys
caused by the movement of underlying
salt layers toward the Colorado
River canyon. The grabens begin
near the Confluence of the Green
and Colorado rivers and run roughly
parallel to Cataract Canyon for
25 km, veering slightly west before
they end. Graben is a German word
meaning ditch or grave. In the geologic
sense it is a collapsed or down-dropped
block of rock that is bordered on
its long sides by faults. Grabens
are normally associated with "horsts,"
which are the up-thrown blocks of
rock. In German, Horst means aerie,
referring to the high nesting sites
of predatory birds.
Geologic
Processes
The processes that led to the development
of the grabens began approximately
300 million years ago in the Pennsylvanian
period with the deposition of evaporates
(salts) in a shallow inland sea.
These deposits, known as the Paradox
member of the Hermosa Formation,
were later covered by the limestone
layers of the upper Hermosa and
Rico formations. In the Needles
District, the Paradox layer can
be 3,000 to 5,000 feet thick.
Sea
levels eventually dropped, and white
sands blew in from the west, forming
large sand dunes. At the same time,
red mud and silt was deposited by
rain and snow melt from the Uncompahgre
Mountain to the east. The resulting
red and white beds alternated, forming
the lower beds of the Cutler Formation,
or the Cedar Mesa Sandstone that
is dominant in the Needles District
today.
Sediment
from a variety of environments continued
to accumulate on top of these layers
for millions of years. Approximately
60 million years ago, a tectonic
plate collision called the Laramide
Orogeny created the Rocky Mountains.
Shortly after, a regional upwarp
called the Monument Uplift caused
the sedimentary layers in the Needles
to tilt gradually westward. This
event also formed joints, or long
parallel fractures in the rock,
throughout the Needles. In the vicinity
of the grabens there are two joint
sets: one trending roughly northeast
to southwest, and one trending northwest
to southeast. Some of these joints
became the faults that border the
grabens.
Around
10 million years ago, the uplift
of the Colorado Plateau gave rise
to the Colorado River and its tributaries.
As the Colorado river cut its way
downward through the rock layers,
it carried away millions of tons
of sediment towards the Pacific
Ocean.
Necessary
Ingredients
Finally, about 55,000 years ago,
the ingredients were in place and
the grabens began to form. Four
factors have been identified as
critical to the formation of grabens:
1.
The ability of evaporates to flow
plastically. The evaporates in the
Needles District are slowly flowing
westward towards Cataract Canyon
due to the pressure exerted by the
rock layers above.
2. The erosion of the Colorado River
down to the Paradox Formation, creating
a low pressure zone the evaporates
are drawn to.
3. The gradual tilt created by the
Monument Uplift, which allows gravity
to act on the evaporates.
4. The interaction of water with
the evaporates, dissolving the salts
and facilitating their ability to
flow.
The
grabens are a very young geologic
feature. Graben growth is thought
to be a slow process where small,
seismically undetectable movement
occurs: as little as one inch per
year. The grabens continue to drop
and slide toward the river today,
and are a fascinating feature of
the Needles District.
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