Wednesday, October 14, 2009

Exploring Vernal Geology by Bike

Note: I first posted this live in April, 2013, three and a half years after I started this post.  I have since moved it back to October 2009, which is closer to when these rides actually happened (Oct 2-4, 2009) 

I had the best of intentions when I started this post, waaaay back in October 2009.  I wanted to do a full-on post about the cool geology under-tire in the Vernal area, but I really only got as far as plotting our rough courses on the geologic maps.  Yesterday, my cousin asked me if we had ever ridden there, so I'm going to throw this post up, hastily, and without proper research.

In October 2009, Steve, Jim, RHandy, Drew, and I road tripped to Vernal, Utah to do some mountain biking on the narrow singletrack Vernal had been getting attention for, like in the June 2009 issue of Bike magazine.

As in many parts of Utah, the geology is well exposed in the Vernal area, and so I tried to take mental notes of what I thought might be the underlying geologic units each time we would traverse some rock ledge or stop at a scenic vista. After our return, I got Drew to show me his Garmin GPS tracks so I could crudely trace them onto the geologic map of the area.Here is what I learned.

Vernal is the heart of the Unita Basin in the Colorado Plateau.  Structurally, it is a basin, or bowl filled with younger and younger sediment toward the center.  Oil and gas are very important extractive industries in the Uinta Basin.  The Unitah and Ouray Indian Reservation is here too.  Water is scarce. 

Day 1 - McCoy Flats area

Upon arriving in Vernal we rode a figure-8 in the McCoy Flats area.

The red, green and blue lines are our rough tracks on the area's geologic map.  Each map unit color represents a different geologic unit from a different time period.  Letters are shorthand for those units or formations.  Lines mostly represent geologic structures such as faults or folds.You can click on this or any other picture in this post to pop up a larger version.

Most of the trails in McCoy Flats are built on the Brennan Basin Member of the Duchesne River Formation (Eocene - 40 million years ago), shown as the peach unit on the geologic map above.  The Brennan Basin Member is made of mostly sandstone and siltstone deposited in river and lake environments, and it has a lot of vertebrate fossils including mammals.  On top of the Brennan Basin Member, shown as the pale yellow on the geo map, is Quaternary alluvium - much younger, unconsolidated to weakly consolidated gravel, cobbles, sand, and silt. 

Here I am riding a trail (hard to see on the photo, as these trails were not very wide, happy day!) on one of the Quaternary alluvium patches draped on the older Tertiary unit.

Siltstone of the Brennan Basin Member erodes into rills.  This trail would be impassable when wet!

After a good little trip around McCoy Flats, it was time to head to the "KOA Kabins" (pic left from the KOA website). The night was cold, but our little space heater kept us toasty in our plastic lined bed.

Day 2 Rojo

After relatively easy trails yesterday, we wanted to try some more technical stuff northwest of town and northwest of Steinaker Reservoir on the Rojo trail. For this, we headed back in (geologic) time.

The Rojo trail starts on Jurassic-Triassic Nugget Sandstone (JTRn - pale green on the geo map below), but the majority of the trail is built on a dip slope of Triassic Chinle Formation (TRc - mint green). The trail is pretty cool. Lots of technical slickrock due to a the resistant sandstone layer that forms the dip slope*.

*A dip slope is a geomorphic feature formed by erosion of softer sediment off a more-resistant, planar unit that is tilted at a low angle.  The resulting surface is the dip slope.

The roughly parallel bands of greens and blues indicate older rocks in the northwest part of the map (the blues and purples) to younger in the southeast part of the map (the light grassy green).  You're looking at the edge of the Uinta Basin.  The center of the basin is to the southeast.  The elongated shape of Steinaker Reservoir is due its location in a strike valley from which softer Jurassic Morrison Formation (Jm) was eroded behind (northwest) of the ridge of Cretaceous sandstones (Kf and Kmd).

The resistant sandstone layer in the Chinle Formation forms a prominent cliff - the trail cuts it pretty close in places! 

Standing on this cliff is standing on the edge of a dip slope of the Chinle Formation.  The softer Moenkopi Formation (TRm) underlies the Chinle.

Here is the view in the other direction looking back at the group.  The cliff edge looks so unreal, but I swear I didn't photoshop this picture.

Day 2 continued: Red Fleet.
After lunch at the Rojo trailhead, we headed over to the Red Fleet area north of town. These trails were super fun.  They weaved in and out of juniper forest and through ravines and over mounds.  Lots of twists and turns. Not too technical, but exciting nonetheless!

Drew on the Red Fleet trails

We rode on fine grained Triassic-aged sediments all afternoon.  The Dinwoody Formation (TRd) is gray or greenish, whereas the Moenkopi Formation is very red. The trails we rode (shown roughly by the blue line on this map) crisscross the contact between the two units several times.  A semi-alert rider will notice the abrupt color change in the dirt over a distance of only a few feet in most places.

This air photo of roughly the same are as the geo map above really shows the difference in outcrop color between the gray and greenish Dinwoody (TRd) and red Moenkopi (TRm)!  Geologists usually map using air photos as a base for their maps - you can see why. 

Drew on a trail on the very red Moenkopi Formation.

And the group traversing the gray Dinwoody.

Group photo. You'd think there was a special on blue Spandex at Walmart!

This is the "Red Fleet", the namesake for the area.  Large red exposures of the Shinarump Conglomerate (a member of the Chinle Fm. TRc) protrude above the landscape, suggesting battleships or the "Red Fleet".

With that sunset on the Red Fleet, we headed back to town for some chow and snooze.

Day 3 - McCoy flats area again.

On our last day we hit the McCoy flats trails again.  This time we rode the green track on the map below.

Click here for Drew's GPS track of the trails we road on Day 3, shown in green on the geo map below.

We found a couple of areas to play in.  I don't remember why I was popping this wheelie.  Probably an accident.

The trails are marked with funny little good trail omens.
More wide open desert on the Duchesne River Formation. 

But rain clouds threatened, so we packed it in and headed for home, waving goodbye to the friendly Vernal dinosaur all decorated for Halloween.

 That was October 2009. We have not been back since - I'm not sure why.  It was definitely worth the trip and Vernal probably has some more "city life" by now. Guess we'd better find some time for another trip.

Sources for this post: 1. The UGS interactive geologic map. 2. "Field Guide to Geologic Excursions in Utah and Adjacent Areas of Nevada, Idaho, and Wyoming" James Wilson ed., GSA Rockky Mtn Section Misc. Pub. 92.3, May 1992.  3. Wikipedia

No comments:

Post a Comment