Saturday, October 28, 2023

An Autumn River Trip Through Desolation and Gray Canyons on the Green River, Utah

During the last week of September, I served as a geologic interpreter on a six day river trip through Desolation and Gray canyons on the Green River in Utah. This was a wonderful time of year as the hordes of mosquitoes had already flown south the fall colors were beginning to grace the cottonwood trees and the temperatures were warm in the day and cool at night. This is a fantastic geologic transect through some relatively young rocks. Have a look.

The trip began with a scenic flight at sunrise to the river - over the canyon we would float

The early morning light was just beginning to fall on the Book Cliffs

The Green River as it exits Desolation Canyon (bottom) and enters Gray Canyon (top). 
Note the large fan coming into the river channel from Three Fords Canyon on the
left. It has pushed the river to the right (west) and remnant bounders washed into the
river form Three Fords Rapid. See a view of this large fan from the ground later in
this post.

View of a cut-off meander along the Green River - we hiked this loop later in the trip

We landed on Horse Mesa Bench, a prominent stratigraphic marker bed
in the Green River Formation

There is a 1.5 mile hike from the landing strip to the river - fabulous!

Great views of the river on our hike - note the Horse Mesa Bench in the middle of the escarpment (small, center cliff face)

Note the hikers (top) and the shadows (bottom)

On the river through the famous Green River Formation

The Green River Formation is a rock unit found in the tri-state area of Utah, Colorado and Wyoming. It represents a series of freshwater lakes that formed in basins that developed on the landscape between about 55 and 45 Ma (Mega-annum, or millions of years ago). At that time the Mogollon Highlands were present in southern Arizona and the Rocky Mountains were being uplifted to the north and east. The basins for the lakes were set between these two positive areas and received sediment from all of the highlands. Prior to a 2010 study, it was unknown that a Mojave Desert source area existed (described below). See a Ron Blakey map next for this lacustrine setting of the Green River Formation.

Paleogeographic map of the Colorado Plateau about 50 Ma

Note the large lakes in the tri-state area. The lake in the middle of the image (the second largest and south of the east/west trending Uinta Mountains) is called Lake Uinta and the photos in this post show the rocks deposited in this particular body of water. The Green River Formation is famous for two reasons – it contains a lot of oil and gas that is being developed in the area, and it holds a spectacularly rich fossil assemblage - if you have ever seen fish fossils for sale in a western rock shop, they likely come from the Green River Formation. Fossil Butte National Monument in Wyoming is the best place to see these and the plants that fringed the shores of the lakes. See this Park Service web page for more information.

Sunrise from camp on the Green River Formation

River reflections

Typical camp scene on beautiful sandy beach

As we moved downstream, the Colton Formation began to emerge from beneath the river

We visited some spectacular petroglyph panels on side hikes

Recall the aerial view of a cut-off meander in the fourth photograph of this post.
Here, we are hiking in the loop of the abandoned meander.

Panoramic view of the abandoned meander near Chicken Rock - see the annotations
next photo

The yellow lines denote the old meander bend (dashed where obscured behind
the meander "island")

The "downstream" part of the Green River's old channel. The entrance and exit of
the meander are very close to the modern bed of he river, suggesting that the
abandonment is relatively recent.

Fantastic view of the Green River in Desolation Canyon. Within the distant wall
of the canyon, note the color difference between the lower, reddish Colton
Formation and the upper, lighter-colored Green River Formation.

Another beautiful camp location - sleeping on these beaches is fantastic!

The Colton is a mostly fluvial and deltaic deposit that slightly pre-dates the Green River
lakes. In the past, it was called the Wasatch Formation but that nomenclature usage is
more restricted to near the Wasatch front 
now. The Colton has an interesting hypothesis 
attached with it.

The Colton Formation is an arkosic sandstone and mudstone (meaning it
contains abundant pink feldspar derived from granitic sources). It also yields
zircon crystals that can be dated to their age of formation. Surprisingly,
the only source for these specific crystals is in the eastern Mojave Desert of
southwestern Arizona and southeastern California. Davis et. al.  postulated
that a continental scale river delivered the sediment from the area of the
Mojave Desert near Blythe to the Uinta basin some 56 to 55 Ma. They called
it the California River (denoted by the thick dashed line). They made no claim
for the exact position of the postulated river, only that it must have flowed
between the various Laramide upwarps (KU=Kaibab upwarp; MU=Monument
upwarp; CCU=Circle Cliffs upwarp; SRS=San Rafael Swell). The scientific
reference is: Davis, Stephen, J., 
(2010) "The Paleogene California River:
Evidence of Mojave-Uinta paleodrainage from U-Pb ages of detrital
zircons", Geol. Soc. of America, Geology, v. 38, no. 10, pp. 931-934.

Note the gradational transition between the reddish Colton Formation (below and
mostly forming
 slopes) and the lighter-colored Green River Formation above (ledges)

As the slope-forming Colton Formation rises higher and becomes more well-developed
the canyon begins to widen out to give much broader views

Above Three Fords Rapid. A large debris fan from Three Fords Canyon pushes the river
to the right (west) and remnants create the largest rapid on the river (not seen).

Camp scene above Three Fords Rapid

Red rock sunset from camp

Majestic buttes composed of the Colton Formation

One last upstream view of Desolation Canyon

The entrance to Gray Canyon - still older rocks emerge - the Mesa Verde Group

Our last nights' camp provided a short hike to a window through the rocks down
to the Green River and our camp

Downstream view in Gray Canyon. The plateau caprock is composed of the Bluecastle
Sandstone Member of the Price River Formation, the upper slope is called the
Buck Tongue; the middle cliff is the Castlegate Sandstone and the lower half of
the view is composed of the Blackhawk Formation. All are Upper Cretaceous.

The trip ended with a view of Gunnison Butte, a famous landmark in the area. This
was a fantastic trip (six days!) with a great group of "students."

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