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The Controversy Over Utah's
Upheaval Dome
written in 1996 by Matt Morgan
October 2006 - Updates are in progress...
For
many years Upheaval Dome, located in the heart of Utah's
Canyonlands National Park, was thought to have been just
an ordinary salt dome. The "salt dome theory" was well
accepted in the scientific community, primarily because
the Canyonlands area contains numerous salt domes.
However, recent stratigraphic and geophysical evidence
suggests a more exciting and controversial origin:
meteorite impact!
Upheaval Dome is 1.5-miles wide from rim to rim and is
over 1/4-mile deep. The rocks contained within the
depression form concentric rings and are the only tilted
strata for miles, thus the name "Upheaval". So what
could cause such an unusual structure? Let's look at the
two most common theories:
Salt Dome Theory
This
theory stems from the possibility that a body of salt
lies beneath Upheaval Dome. 300 million years ago, a
vast inland sea covered the Canyonlands region (Figure
1, Illustration A).
Over
millions of years the sea evaporated, leaving a large
deposit of salt (Illustration B). Sediments were
deposited over the salt, forming thick layers of rock
(Illustration C). The overburden weight from the rocks
pushed down on the salt. Since the salt is less dense
than the rocks above, it becomes buoyant. Pushing up the
overlying layers of rock, the salt forms a dome
(Illustration D). Erosional processes remove the
overlying layers of rock, exposing the tilted strata
below. A "crater" is formed (Illustration E)
Meteorite Impact
Theory
Only
until recently has the idea that a meteorite impact
created Upheaval Dome been taken seriously.
Approximately 60 million years ago, a large body
1/3-mile in diameter, smashed into this area and created
a large meteorite crater (Figure 2, Illustration A , B,
and C).
The
upturned beds found rimming the crater were produced
when the rocks rebounded and extended from the
tremendous force of the impact (Illustration D). Over
millions of years, erosional processes removed much of
the overlying layers of rock. Upheaval Dome is the
remnant of the lower portions of the crater, with
fragments of the impacting body completely gone
(Illustration E).
The Evidence
There
are several key pieces of evidence that point to a
meteorite impact for the origin of Upheaval Dome. In
1993, Eugene Shoemaker and Ken Herkenhoff found shatter
cones within the sandstone of Upheaval Dome.
Shattercones, as their name implies, are conical-shaped
pieces of rock that have small grooves radiating from a
central point (Figure 3).
It is
believed that shatter cones form when shockwaves from a
meteorite impact move through the surrounding rock,
leaving the grooves behind. Shoemaker and Herkenhoff
also found shocked quartz, which is ordinary quartz that
has been highly fractured from an impact, within the
sandstone. Even more evidence for the impact theory was
presented in 1995 by John Louie, a seismologist at the
University of Nevada-Reno. Louie set out to determine if
there is a rising salt plume beneath the surface of
Upheaval Dome. Louie and his colleagues used a technique
called seismic refraction, where a seismic wave is sent
nearly parallel to a series of high velocity rocks and
the travel time of the seismic wave is recorded. From
the return time, the thickness of the underlying rocks
can be determined. If Upheaval Dome was created by a
rising salt plume, the rocks should show more
deformation with increasing depth. However, if Upheaval
Dome was created by a meteorite impact, the rocks will
be deformed only near the surface. Louie found that the
velocity of the seismic waves do not slow down (as they
would through salt) as they pass through the rocks
beneath the structure. This suggests, almost
conclusively, that no salt dome exists beneath Upheaval
Dome. "Upheaval Dome is the remnant of a meteorite
impact because everything underneath it looks like it
was punched out by a force from above", Louie said in
the Salt Lake Tribune. Add the seismic evidence to the
shocked-quartz and shatter-cones found at Upheaval Dome
and you have a "classic" meteorite crater.
Further Study
Continuing their NASA-funded project, Shoemaker and
Herkenhoff are mapping the rocks that make-up Upheaval
Dome. They found that some of the strata beneath the
dome are thicker than others. Both believe this is
central uplift of the crater, formed by the rebound of
the rocks after impact. Shoemaker also believes the
original crater was 3 miles wide and was formed by a
comet or asteroid 1000-feet in diameter. It is possible
that further mapping may reveal the actual size of this
catastrophic event. Upheaval Dome, is a really exciting
place located in Utah's Canyonlands National Park.
Definitely, a site that should not be missed!
Matt Morgan
Mile High Meteorites
http://www.mhmeteorites.com
References
Louie, John. 1995.
"Upheaval Dome Seismic Refraction Analysis". On-line
document at:
http://www.seismo.unr.edu/ftp/pub/louie/dome/refraction.html
Siegel, Lee. 1995.
"NASA: Big Meteorite Whacked Utah". The Salt Lake
Tribune.
I would also like to
thank the National Park Service and the Canadian
Department of Natural Resources.
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