Paleogeography (Historical Geology) Research
Appendix to the Evolution of the Colorado River and its Tributaries (Part 7)
Colorado's Gore Range
by
Bill Butler
This mountain range (and
the area northeast of it) is significant as it reveals the early
history of the headwaters of the Colorado River. The range has several
curiosities including two major canyons that cut through it. Tenmile
Creek cut Tenmile Canyon and the Colorado River cut Gore Canyon. A
major question has been – How did these streams get across the
Gore Range to form the canyons? It has been assumed that the range was
uplifted during the Laramide. However, this makes it very difficult to
form these breaches. The problem becomes much easier if we can show the
present Gore Range did not rise until Miocene time. (Recent research
implies much of this uplift has occurred in the last 5 million years.)
This allows lots of time for both of these streams to be in place
before the range rose.
First, let’s look at some of the unusual/unexpected phenomena that exist in the range.
1) Tenmile Creek cuts a SSW to NNE canyon through the range at about the 9,100 to 9,600 foot level. (Highway I-70 uses this canyon to get through the range.) Peaks exceed 12,000 feet within two miles on both sides of Tenmile Canyon.
2) The Colorado River has cut westward through the range at Gore Canyon. The floor of the canyon is at about 7,200 feet while the rim on both sides exceeds 9,300 feet within a mile of the river.
3) A dry abandoned valley (similar to Unaweep Canyon) cuts across the range a couple of miles south of Gore Canyon. At some time in the past a tributary to the Colorado River flowed east to west across the crest of the range through this abandoned valley.
4) Another high dry valley cuts across the range some 6 miles northwest of Gore Canyon at Gore Pass. At some time in the past, another tributary crossed the range from northeast to southwest through this abandoned valley. (The formation of this valley was probably helped by a NE-SW fault.)
5) Both Gore Creek and Service Creek start down the east side of the range and then double back through the crest to flow down the west side. In both cases, these streams use segments of ancestral rivers that crossed the crest of the range from east to west.
6) The heart of the Gore Range just north of Tenmile Canyon has a jagged, “young-looking” appearance as compared to the rounded Laramide age Front Range just 15 miles further east.
If we look at the area bounded by the Gore/Park Range to the west and the Medicine Bow and Front Ranges to the east, we find a broad area covered by early Tertiary deposits. The east to west Rabbit Ears Range splits this into North and Middle Parks although the Tertiary deposits wrap over the top of the range. Also there are splatterings of mid/late Tertiary volcanics across the top and to south of the Rabbit Ears Range. The Gore/Park Range itself has no Tertiary deposits except for a couple of Tertiary intrusions and some Tertiary lava flows near the west end of the Rabbit Ears Range.
The main feature of the Gore Range is Gore Canyon. Here, the Colorado River cuts a short, deep (over 2,000 feet), young-looking gorge across the crest of the range. If you smoothed out the contours across the gorge, the crest would be about 9,400 feet above sea level. Muddy Creek is a tributary that feeds into the Colorado before it cuts through Gore Canyon. At the north end of Muddy Creek’s drainage (off the top of the view) there are multiple potential exit routes that are less than 9,000 feet including Muddy Pass at slightly over 8,700 feet. To the east of Muddy Pass we have early Tertiary sediments and mid/late Tertiary lava flows overlying Cretaceous layers and still stay well under 9,400 feet. The Tertiary deposits indicate this area has never been over 9,400 feet.
There are two possible ways the Colorado River could have established a path across the Gore Range. Either (1) at some time in the past there was a higher sedimentary or pediment surface east of the range, or (2) the Colorado River established its westward course before there was a (significant) uplift of the range. It has always been assumed that case number (1) was true. However, given that alternate exit routes that stay under 9,400 feet have always been available, the evidence seems to support alternative (2). Thus, we offer the following model.
During the initial Laramide uplifts, the Front Range was lifted to high
elevations and the current Gore/Park Range was only lifted slightly.
(Just enough so that the top sedimentary layers would erode.) In
between the two ranges, the headwaters of the ancestral Colorado flowed
north into Wyoming. Tenmile Creek (southwest of Dillon, Co) and the
Blue River are part of this ancestral drainage. The most likely course
of this ancestral river would be a continuation of the Blue River from
Williams Fork Reservoir north-northeastward to just southeast of Willow
Creek Pass. Here it joined other drainage flowing down from the west
slopes of the Front Range. The combination then turned
north-northwestward and continued into Wyoming. This is the area where
the thickest (over 2,000 feet) Paleocene/Eocene deposits are found.
During the Paleocene and Eocene, the earlier minor uplift that had been
present in the Gore/Park Range eroded down to where it was only
slightly more elevated than these river deposits.
During the late Oligocene /early Miocene, there was a general uplift across southern Wyoming and local uplift of the Rabbit Ears Range. The early Tertiary deposits were also capped by local lava flows. The eastern end of the Rabbit Ears Range was lifted several thousand feet. Gravel deposits from the old riverbed can now be found at 11,700 feet on Gravel Mountain (some 7 miles southeast of Willow Creek Pass). The old Paleocene/Eocene deposits are now found at elevations ranging from 8,000 to 11,000 feet.
This uplift forced the ancestral Colorado to find a new route. It
turned west-southwest to establish its current course over the top of
the beveled Laramide version of the Gore Range. The Blue River turned
west just south of the current Gore Canyon and joined the Colorado at
the west end of the present canyon. Other steams flowing westward off
the southwest side of the new Rabbit Ears Range established paths that
would be used later by Service, Silver, and Gore Creeks when the recent
Gore Range rose.
After all this new drainage was established, the Gore Range rose during the Miocene. The Colorado River was in place and proceeded to cut Gore Canyon. (The river’s steep gradient through the canyon implies the range continues to slowly rise today.) The Blue River continued its path across the range (just south of the current Gore Canyon) for awhile but eventually found an easier route staying east of the range until it joined the Colorado. (“Let the big guy do the canyon cutting”.) The Blue River’s old route across the Gore Range was abandoned. However this old high, dry valley is still used by highway CR 1 (Trough Rd.) to get across the range.
Meanwhile, to the northwest of Gore Canyon, the small streams that previously flowed westward from the southwest side of the Rabbit Ears Range were forced to find new routes. As the Gore Range rose, these old streams were broken off on the northeast facing slopes of the rising range, as this is where the greatest change in tilt was taking place. The new local drainage of streams such as Service, Silver, and Gore Creeks appropriated the ancestral paths across the crest of the range. For example, Service Creek starts down the east side of the range. However, it has appropriated a section of an ancestral stream that crossed from east to west before the range was uplifted. Service Creek uses this old segment to double back through the crest of the range to flow down the west side.
A new stream, Muddy Creek, developed between the rising Gore Range and the Rabbit Ears Range. It flowed south-southeast for awhile and then it turned southwest to cross the Gore Range at Gore Pass. (A northeast/southwest fault helped by uplifting the southeast side of the valley.) However, similar to the Blue River, Muddy Creek decided it was easier to “Let the big guy do the canyon cutting” and abandoned the high route. Thus, it rerouted further southeast to join the Colorado east of Gore Canyon. Highway builders are always looking for the easiest routes to get across mountain ranges, and Colorado State highway 134 uses the old, abandoned high valley.
The Gore Range rose through most of the Miocene (and probably is still rising), but the Colorado River had established its course. Gore Canyon is the result.
Also please see Origin and Formation of Gore Canyon, Colorado
First, let’s look at some of the unusual/unexpected phenomena that exist in the range.
1) Tenmile Creek cuts a SSW to NNE canyon through the range at about the 9,100 to 9,600 foot level. (Highway I-70 uses this canyon to get through the range.) Peaks exceed 12,000 feet within two miles on both sides of Tenmile Canyon.
2) The Colorado River has cut westward through the range at Gore Canyon. The floor of the canyon is at about 7,200 feet while the rim on both sides exceeds 9,300 feet within a mile of the river.
3) A dry abandoned valley (similar to Unaweep Canyon) cuts across the range a couple of miles south of Gore Canyon. At some time in the past a tributary to the Colorado River flowed east to west across the crest of the range through this abandoned valley.
4) Another high dry valley cuts across the range some 6 miles northwest of Gore Canyon at Gore Pass. At some time in the past, another tributary crossed the range from northeast to southwest through this abandoned valley. (The formation of this valley was probably helped by a NE-SW fault.)
5) Both Gore Creek and Service Creek start down the east side of the range and then double back through the crest to flow down the west side. In both cases, these streams use segments of ancestral rivers that crossed the crest of the range from east to west.
6) The heart of the Gore Range just north of Tenmile Canyon has a jagged, “young-looking” appearance as compared to the rounded Laramide age Front Range just 15 miles further east.
If we look at the area bounded by the Gore/Park Range to the west and the Medicine Bow and Front Ranges to the east, we find a broad area covered by early Tertiary deposits. The east to west Rabbit Ears Range splits this into North and Middle Parks although the Tertiary deposits wrap over the top of the range. Also there are splatterings of mid/late Tertiary volcanics across the top and to south of the Rabbit Ears Range. The Gore/Park Range itself has no Tertiary deposits except for a couple of Tertiary intrusions and some Tertiary lava flows near the west end of the Rabbit Ears Range.
The main feature of the Gore Range is Gore Canyon. Here, the Colorado River cuts a short, deep (over 2,000 feet), young-looking gorge across the crest of the range. If you smoothed out the contours across the gorge, the crest would be about 9,400 feet above sea level. Muddy Creek is a tributary that feeds into the Colorado before it cuts through Gore Canyon. At the north end of Muddy Creek’s drainage (off the top of the view) there are multiple potential exit routes that are less than 9,000 feet including Muddy Pass at slightly over 8,700 feet. To the east of Muddy Pass we have early Tertiary sediments and mid/late Tertiary lava flows overlying Cretaceous layers and still stay well under 9,400 feet. The Tertiary deposits indicate this area has never been over 9,400 feet.
There are two possible ways the Colorado River could have established a path across the Gore Range. Either (1) at some time in the past there was a higher sedimentary or pediment surface east of the range, or (2) the Colorado River established its westward course before there was a (significant) uplift of the range. It has always been assumed that case number (1) was true. However, given that alternate exit routes that stay under 9,400 feet have always been available, the evidence seems to support alternative (2). Thus, we offer the following model.
During the initial Laramide uplifts, the Front Range was lifted to high
elevations and the current Gore/Park Range was only lifted slightly.
(Just enough so that the top sedimentary layers would erode.) In
between the two ranges, the headwaters of the ancestral Colorado flowed
north into Wyoming. Tenmile Creek (southwest of Dillon, Co) and the
Blue River are part of this ancestral drainage. The most likely course
of this ancestral river would be a continuation of the Blue River from
Williams Fork Reservoir north-northeastward to just southeast of Willow
Creek Pass. Here it joined other drainage flowing down from the west
slopes of the Front Range. The combination then turned
north-northwestward and continued into Wyoming. This is the area where
the thickest (over 2,000 feet) Paleocene/Eocene deposits are found.
During the Paleocene and Eocene, the earlier minor uplift that had been
present in the Gore/Park Range eroded down to where it was only
slightly more elevated than these river deposits.During the late Oligocene /early Miocene, there was a general uplift across southern Wyoming and local uplift of the Rabbit Ears Range. The early Tertiary deposits were also capped by local lava flows. The eastern end of the Rabbit Ears Range was lifted several thousand feet. Gravel deposits from the old riverbed can now be found at 11,700 feet on Gravel Mountain (some 7 miles southeast of Willow Creek Pass). The old Paleocene/Eocene deposits are now found at elevations ranging from 8,000 to 11,000 feet.
This uplift forced the ancestral Colorado to find a new route. It
turned west-southwest to establish its current course over the top of
the beveled Laramide version of the Gore Range. The Blue River turned
west just south of the current Gore Canyon and joined the Colorado at
the west end of the present canyon. Other steams flowing westward off
the southwest side of the new Rabbit Ears Range established paths that
would be used later by Service, Silver, and Gore Creeks when the recent
Gore Range rose.After all this new drainage was established, the Gore Range rose during the Miocene. The Colorado River was in place and proceeded to cut Gore Canyon. (The river’s steep gradient through the canyon implies the range continues to slowly rise today.) The Blue River continued its path across the range (just south of the current Gore Canyon) for awhile but eventually found an easier route staying east of the range until it joined the Colorado. (“Let the big guy do the canyon cutting”.) The Blue River’s old route across the Gore Range was abandoned. However this old high, dry valley is still used by highway CR 1 (Trough Rd.) to get across the range.
Meanwhile, to the northwest of Gore Canyon, the small streams that previously flowed westward from the southwest side of the Rabbit Ears Range were forced to find new routes. As the Gore Range rose, these old streams were broken off on the northeast facing slopes of the rising range, as this is where the greatest change in tilt was taking place. The new local drainage of streams such as Service, Silver, and Gore Creeks appropriated the ancestral paths across the crest of the range. For example, Service Creek starts down the east side of the range. However, it has appropriated a section of an ancestral stream that crossed from east to west before the range was uplifted. Service Creek uses this old segment to double back through the crest of the range to flow down the west side.
A new stream, Muddy Creek, developed between the rising Gore Range and the Rabbit Ears Range. It flowed south-southeast for awhile and then it turned southwest to cross the Gore Range at Gore Pass. (A northeast/southwest fault helped by uplifting the southeast side of the valley.) However, similar to the Blue River, Muddy Creek decided it was easier to “Let the big guy do the canyon cutting” and abandoned the high route. Thus, it rerouted further southeast to join the Colorado east of Gore Canyon. Highway builders are always looking for the easiest routes to get across mountain ranges, and Colorado State highway 134 uses the old, abandoned high valley.
The Gore Range rose through most of the Miocene (and probably is still rising), but the Colorado River had established its course. Gore Canyon is the result.
Also please see Origin and Formation of Gore Canyon, Colorado