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Abstract
Unaweep Canyon is the dominant feature of the Uncompahgre Plateau.
The origin of this spectacular wind gap has been attributed to a large
river. Most Geologists feel that the Gunnison and Colorado Rivers together
carved out Unaweep Canyon. However, this idea was based on very little
physical evidence. It is my contention that Cactus Park is a key element
in determining the origin of Unaweep Canyon. I have conducted pebble count
data from the Colorado River, Gunnison River, and Cactus Park. This data
shows that the Gunnison River once occupied Unaweep Canyon. My findings
do not reveal the complete history of Unaweep Canyon, but they are an important
piece of the overall picture. My data also gives an important glimpse into
what the Gunnison River was like during Cactus Park time.
Introduction
Unaweep Canyon is a remarkable physiographic feature that crosses the
axis of the Uncompahgre Plateau (Figure 1). The origin of Unaweep Canyon
has been the subject of considerable conjecture and debate since the Hayden
Survey first described the feature more than 100 years ago (Gannett, 1882).
A variety of proposals have been submitted for the origin of this canyon,
but only a small amount of data exists in support of the various hypotheses.
Many geologists have attributed the cutting of the canyon to the ancestral
Gunnison or Colorado Rivers working separately or together during the late
Tertiary (Cole and Young, 1983). This belief exists primarily because of
the gravel deposits located in Cactus Park. Cactus Park connects onto the
east side of Unaweep Canyon eight miles from the town of Whitewater, Colorado.
It is in Cactus Park that the best evidence for the origin of Unaweep Canyon
is located. Ancient river gravels in Cactus Park support the theory that
the Gunnison River once occupied the canyon. However, it must be noted
that the geomorphic history of Unaweep canyon is complicated, and there
are other possible factors that must be considered in the evolution of
Unaweep Canyon. In this paper I will describe the composition and interpret
the provenance of Cactus Park gravel, and discuss the possible scenarios
for the origin of Unaweep Canyon. The method I will use to show the provenance
of Cactus Park gravel is a comparison of pebble counts from the modern
Gunnison and Colorado Rivers to pebble count data from Cactus Park. It
is my hope that this paper will add to the geologic knowledge of Unaweep
Canyon in the hope of a better understanding of this spectacular geologic
feature.
Description of Unaweep Canyon and Cactus Park
Unaweep Canyon is a northeast – southwest trending canyon that cuts
across the axis of the Uncompahgre Plateau between Whitewater, and Gateway,
Colorado. The total length of the canyon is 43 miles. The canyon is 4.3
miles wide and over 3000 ft deep. The elevation at the northeastern mouth
of the canyon near Whitewater is around 5000 ft, and the elevation at the
southwestern mouth of the canyon near Gateway is also around 5000 ft. The
canyon floor has an elevation of 7120 ft at its summit, and there is over
2000 ft of relief present in Unaweep Canyon (Figure 2, Table 1). Based
on differences in rock type and topographic gradient the canyon can be
broken up into four segments – west, central, Cactus Park, and east (Cole
and Young, 1983).
The west section of Unaweep Canyon extends from Gateway, Colorado to the eastern tip of the Ute Creek Graben. The west section contains two areas of vastly different rocks: the western Cutler section and the eastern Precambrian rocks of the Ute Creek Graben. The Cutler section is characterized by Pleistocene fanglomerates on top of the Permian Cutler Formation. The topographic gradient in the Cutler section is 94 ft/mi. In the Graben the topographic gradient is over 200 ft/mi, and an impressive steep walled gorge exposes 1400 ft of Precambrian rock.
The central section of Unaweep canyon is 24 miles long and is cut through Precambrian crystalline rocks. The central section can be divided further into the section west of the divide (West Creek), and a section east of the divide (East Creek). The gradient west of the divide is less than on the eastern side of the divide, and the canyon also widens dramatically west of the divide. Unaweep Canyon reaches its maximum width (4.3 mi) and depth (3000 ft) near the western boundary of the central section. The central section of Unaweep Canyon is generally broad, flat, and U-shaped. Precambrian schist, gneiss, and pink pegmatite dikes characterize the rocks of the central section (Mose and Bickford, 1969).
The rock types of Cactus Park are the same Jurassic and Cretaceous sandstones, siltstones and shales as in the east section of Unaweep Canyon. The topographic gradient however, is nearly the same as the central section. It is for those reasons that I have designated Cactus Park as an independent section of Unaweep Canyon. Cactus Park is a broad valley that trends northwest and intersects Unaweep Canyon eight miles southwest of the town of Whitewater (Figure 1). The elevation of Cactus Park is 6100 ft, and it is believed to be an abandoned channel of the Gunnison River (Cater, 1966). There is an ancient gravel deposit in Cactus Park, and this gravel is the focus of my pebble count studies.
The east section of Unaweep Canyon is carved in sandstones, siltstones and shales of the Jurassic Morrison Formation and the Cretaceous Burro Canyon Formation and Dakota Sandstone. This section has an average gradient of about 200 ft/mile (Table 1) and the canyon is relatively broad and shallow. From rim to rim the canyon is nearly a mile wide, but only 500-ft deep. The east section of Unaweep also contains some fascinating landslide deposits. Huge blocks of Cedar Mountain sandstone rest on slumping shale of the Brushy Basin Member of the Morrison Formation. The chaotic surface of this landslide deposit is visually striking, and was produced by frost wedging and solifluction during the early Holocene (Cole and Young, 1983).
Methods
In this section I will describe the methods I used to take pebble count
data in Cactus Park, and from various sites around the Grand Valley (Figures
4-7, and Tables 2-9). To start with it must be noted that there was existing
pebble count data before I started this project. The Fall 2000 Mesa State
College Geomorphology class compiled pebble count data for the Colorado
River near the Butterfly Museum (Figure 4). Also, David Lutz took a comprehensive
set of pebble counts from the Colorado River, Gunnison River, and the Colorado
River below its confluence with the Gunnison (Lutz, 1999, and Table5).
My first task was to visit each of these sites, and aquatint myself with
the various rock types described in the existing data. It must be noted
that D. Lutz, the Geomorphology class, and myself did not initially agree
on what to label each rock type. Dr. Aslan and I decided that the best
course of action would be to gather representative rocks from each river,
and together come up with a consistent set of rock names for the existing
data. This was accomplished, and data for the modern Colorado River was
generated (Table 2-9). After this was accomplished I set out to record
my own pebble count data. I chose to take new pebble count data near Whitewater,
and also in Escalante Canyon, both of which are low terraces on the modern
Gunnison River. Next, I took two different pebble counts in Cactus Park
(Figures 8, 9). It was later discovered that one of my pebble counts from
Cactus Park was actually a landslide deposit with some reworked river gravel
in it, so I only used one pebble count from Cactus Park. It should be noted
that each pebble count consisted of around 250 pebbles at each location
(Tables 2-9).
Results From the Pebble Count Data
Figure 10, is from the data from Tables 2-9, and shows a comparison
of igneous, sedimentary, and metamorphic clasts from Cactus Park and from
the Colorado and Gunnison Rivers. The results from this data overwhelming
states that Cactus Park is a Gunnison River deposit. If Cactus Park was
once a tributary of Unaweep Canyon, then it can also be concluded that
the Gunnison River once occupied Unaweep Canyon. Figure 11, shows a more
in depth look at the clasts from Cactus Park compared to those from the
Gunnison River. This more detailed look illustrates that the gravel deposit
in Cactus Park is exclusively from the Gunnison River.
There are other implications that are brought out by the Cactus Park
Pebble count data. First, the amount of black metasediment (shale) that
is in the Cactus Park gravel is very close to the amount in current Gunnison
River gravels (Figure 12). This implies that the North Fork of the Gunnison
River was contributing sediment to the Gunnison River during Cactus Park
time (Lohman, 1981). Also, the presence of schist, gneiss, and quartzite
in Cactus Park implies that the Gunnison River was flowing through the
Black Canyon during the occupation of Unaweep. The presence of gray-purple
volcanic tuff suggests that the Uncompahgre River was contributing sediment
to the Gunnison River, as it is today The overall implication of this data
is that the drainage pattern of the Gunnison River was very much like it
is today .
Uplift of Unaweep Canyon
This paper is based on the proposition that a large river once occupied
Unaweep Canyon. However, this could not be a true unless significant uplift
had occurred between the time Unaweep Canyon was abandoned and the present.
Cactus Park is at an elevation of around 6100 ft, which is over 1000 ft
of elevation lower than the divide of Unaweep Canyon (Figure 2). Rivers
do not flow up hill, so there are two possibilities for the discrepancy
in elevation. Possibility one is that there is over 1000 ft of fill in
the bottom of Unaweep Canyon, and possibility two is that there has been
at least 1000 ft of uplift since its abandonment by the Colorado/Gunnison
River. Oesleby, 1973 did a graduate thesis on fill thickness in Unaweep
Canyon. He used seismic refraction on the canyon floor in an attempt to
discover the depth of fill in Unaweep Canyon. The conclusion of his work
was that there was indeed almost 1000 ft of fill in the bottom of Unaweep
Canyon (Oesleby, 1973). However, it must be noted that all of his refraction
lines were run parallel to the canyon axis and his geophone spread was
relatively short. This means that his data is has echoes from the canyon
walls, which may have mistakenly lead to the conclusion that there is 1000
ft of fill at the Unaweep Divide. There is very little evidence for the
exact bedrock depth of Unaweep Canyon. There have been a number of agricultural
water wells drilled in Unaweep Canyon, but records for these wells are
poor to non-existent. The bottom line is that we are still unsure about
the depth of valley fill in Unaweep Canyon. If uplift did occur, the probable
mechanisms were faulting, and/or isostatic rebound. There are faults bounding
the Uncompahgre Plateau that could accommodate such uplift, but I know
of no work that has been done to prove this idea. The idea of isostatic
rebound is also plausible, but that topic is well beyond the scope of my
paper. The debate over uplift vs. valley fill is worthy of attention, but
it would take a very expensive drilling project to definitively prove which
idea is correct.
Possible Scenarios for the Origin of Unaweep Canyon
Two small streams currently drain Unaweep Canyon: East Creek, and West
Creek. These streams are seasonal, and underfit for the task of cutting
a canyon the size and dimensions of Unaweep Canyon. Most geologists that
have studied Unaweep Canyon have concluded that it was once occupied by
a major river (Lohman, 1981; Hunt, 1956). However, few geologists agree
on which river contributed to the origin of Unaweep Canyon. There are at
least four different scenarios that I have considered for the origin of
Unaweep Canyon.
The first scenario involves Unaweep Canyon being cut exclusively by the Colorado River. The idea that the Colorado River once flowed through Unaweep Canyon is based primarily on the alignment of the canyon with the current course of the Colorado River above Grand Junction. The ancient gravel deposits at Cactus Park suggest that the Gunnison River was also in Unaweep Canyon, and so this theory is not very viable. There are geologists that have supported this idea (Gannett, 1882), but there is not a lot of good evidence for this theory. The fact that this theory excludes the entire drainage of the Gunnison and Uncompahgre Rivers is unacceptable to me.
The second possible scenario for the origin of Unaweep Canyon is that the Gunnison River cut it without the Colorado River ever being involved (Lohman, 1961). This theory has much more evidence in its support. The first and best evidence for this theory is the gravel deposit of Cactus Park. I have shown earlier in this paper that the provenance of Cactus Park gravel is the exclusively the Gunnison River. The second reason for believing that the Colorado River was never in Unaweep Canyon is the valley morphology of East Creek north of Cactus Park. The canyon of East Creek north of Cactus Park is relatively narrow, and the topographic gradient is steep (203 ft/mi). Unaweep Canyon above Cactus Park has the appearance of a relatively unmodified channel of a large river, but the appearance of East Creek Canyon north of Cactus Park is of a canyon cut by a small stream that was never occupied by a large river (Cater, 1966). However, incision in East Creek since the possible occupation by the Colorado River could explain its current appearance.
The third possible scenario for the origin of Unaweep Canyon has both the Colorado River and the Gunnison River cutting the canyon (Sinnock, 1981b). Proponents of this idea point to the gravel deposits of Cactus Park as evidence for occupation by the Gunnison River, and point out that the Colorado River may have never passed through Cactus Park. The idea is that the Colorado River occupied East Creek, and all evidence for this occupation has been eroded. Lohman (1981) developed a four-stage model for the development of Unaweep Canyon (Figure 3). He proposed that during stage 1, the Colorado and Gunnison Rivers occupied Unaweep Canyon. In stage 2, the Colorado River was captured and diverted to its present course, leaving the Gunnison River to occupy Unaweep Canyon. In stage 3 uplift of the Uncompahgre Plateau caused the Gunnison River to abandon Unaweep Canyon and rejoin the Colorado River. Finally, in stage 4, Unaweep Canyon experiences further uplift and the present day drainage patterns are now in effect.
The forth-possible scenario for the formation of Unaweep Canyon differs dramatically every other idea, and has the Dolores and San Miguel Rivers occupying Unaweep Canyon. If this theory were correct, then the flow direction in Unaweep Canyon would have been from the south, and would have the San Miguel River emptying into the Gunnison River somewhere near Whitewater (Hunt, 1956). There is very little existing data to support this idea, and it would take a discovery of San Miguel River gravel in the Grand Valley to convince me that this is a viable theory. However, if one looks at the possible uplift that has occurred in the Uncompahgre Plateau and the topographic gradients of the San Miguel River, then this theory is not totally impossible. It should also be noted that it would be worth while to check along the terraces of the Dolores River for Colorado/Gunnison River deposits.
Glacial Influence in Unaweep Canyon
The last peace of the geomorphic history of Unaweep Canyon is the possible
influence of glacial ice. Unaweep Canyon does not exhibit the fresh glacial
features on might expect from Wisconsin alpine glaciation, but there are
some features in Unaweep Canyon that could be due to glaciers. The most
obvious feature of Unaweep Canyon is its broad U-shaped morphology. Many
geologist that have worked in Unaweep suggest that the canyon is actually
V-shaped with lots of fill in the bottom (Oesleby, 1978). Fill depth of
Unaweep Canyon has been discussed earlier in this paper, but it should
be restated that the exact amount of fill depth is uncertain. Cole and
Young (1983) suggest that Unaweep Canyon was once glaciated. In that paper
they have described the existence of truncated spurs, cirques, hanging
valleys, and glacial till in Unaweep Canyon. The most interesting glacial
feature noted by Cole and Young (1983) is the large moraine they described
in West Creek near The Seeps. If this feature were an actual moraine, it
would mark the furthest advance of ice in West Creek. One puzzling thing
is that no glacial outwash deposit has ever been mapped in East Creek (Sinnock
1981a). I do not feel that the origin of Unaweep Canyon can be completely
attribute to glaciation, but I think that there has been at least minor
glacial influence in Unaweep Canyon.
Conclusions
From the data that I have gathered, I believe the provenance of Cactus
Park Gravel is the Gunnison River. And so, I conclude that the Gunnison
River at one time occupied Unaweep Canyon. From my data, I cannot make
any other definite conclusions about the origin of Unaweep Canyon. It would
be imprudent to conclude that the Colorado River could not ever have occupied
Unaweep Canyon, solely on my pebble count data from Cactus Park. The fact
is, there is no evidence placing the Colorado River in Unaweep Canyon,
but there is no evidence to show that it was never there. I find myself
in the same place as many who have studied this canyon, in that, I have
only my ideas as to what happened. However, my data are useful in showing
what the drainage of the Gunnison River was like in Cactus Park time. My
data shows that the drainage of the Gunnison River was very similar then,
and it is today. It is my hope that continued work will be done in Unaweep
Canyon, and with the history of the Gunnison and Colorado Rivers.
Bibliography
Cater, F.W., 1966, Age of the Uncompahgre Uplift and Unaweep Canyon, West-Central Colorado, U.S. Geological Survey Prof. Paper 550-C.
Cole, R. D. and Young, R. J., 1983, Evidence for Glaciation in Unaweep
Canyon,
Mesa County, Colorado, Grand Junction Geological Society – 1983 Field
Trip Paper,
p. 73-80.
Gannett, H., 1882, The Unaweep Canyon, Popular Science Monthly, V. 20, p. 781-786.
Hunt, C.B., 1956, Cenozoic Geology of the Colorado Plateau, U.S. Geological Survey Paper 279.
Lohman, S. W., 1961, Abandonment of Unaweep Canyon, Mesa County, Colorado,
By capture of the Colorado and Gunnison Rivers in Short papers in the Geologic
and Hydrologic Sciences, Articles 1- 146, U.S. Geological Survey Professional
Paper 424-B
p. 59-130.
Lohman, S. W., 1981, Ancient drainage changes in and south of Unaweep Canyon, southwestern Colorado, in Epis, R.C., and Callender, J.F., eds., Western Slope Colorado, New Mexico Geological Society 32nd Field Conference, p. 137-143.
Lutz, D., 2000, Gravels of the Valley, A comparison of the Colorado and Gunnison River, Pebble Count Data for Geomorphology Class, Mesa State College, Unpublished Undergraduate Paper.
Mose, D. and Bickford, M., 1969, Precambrian Geochronology in the Unaweep Canyon, West-Central Colorado, Journal of Geophysical Research, V. 74, p. 1677-1687.
Oesleby, T. W., 1973 Uplift and Deformation of the Uncompahgre Plateau: Evidence from fill thickness in Unaweep Canyon, West-Central Colorado, Thesis for the University of Colorado
Sinnock, S., 1981a, Glacial moraines terraces and pediments of Grand Valley, western Colorado, in Epis, R.C., and Callender, J.F., eds., Western Slope Colorado, New Mexico Geological Society 32nd Field Conference, p. 113 –120.
Sinnock, S., 1981b, Pleistocene drainage changes in Uncompahgre Plateau-Grand
Valley region of western Colorado, including formation and abandonment
of Unaweep Canyon: a hypothesis, in Epis, R.C., and Callender, J.F., eds.,
Western Slope Colorado, New Mexico Geological Society 32nd Field Conference,
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