Site 23

After six days amidst the maroon, red, pink, orange, tan and white sedimentary rocks that dominate the landscapes of  the Colorado Plateau Province, I woke up this morning to something very different.

Monday morning with Navajo sandstone at the Wedge Overlook, San Rafael Swell, Colorado Plateau.

Tuesday morning with Sevier River hoodoos, Tushar Mountains, eastern Basin and Range Province.

My neighbors here in the Castle Rock Campground are hoodoos eroded from the mid-Tertiary Sevier River Formation, a mix of sedimentary strata derived from volcanic rocks upstream.  The volcanics are products of the Mt. Belknap Caldera ca 10 miles to the south, part of the massive volcanic conflagration in the Basin and Range Province in mid-Tertiary time.

Volcanic material was eroded off the highlands and deposited in lake beds, canyons and river channels, including the one now occupied by Castle Rock Campground.  Then, with subsequent Basin and Range uplift, streams were rejuvenated and the river deposits themselves were eroded -- forming these wonderful hoodoos.  The layered texture reflects sediment diversity, ranging from fine to quite coarse, with some cobbles up to several feet across.

Bands of cobbles deposited during more energetic river flow ... perhaps flash floods?

Ash layers (white bands) indicate there was ongoing volcanic activity in the area during deposition.  Note the gradation of color above the lower ash layer with deposits containing progressively less ash (vs. the sharp contrast below).  A younger layer is visible near the top of the highest castle in upper right of photo.  These two layers have been dated at 13.8 and 6.9 Ma.

Most of the material of the Sevier River Formation was derived from the Joe Lot tuff, which covers much of the area.  The contact between the two rises above the upstream end of the campground.  Rises above???  That’s because the formations are severely tilted, even though Sevier River strata are nearly horizontal just a short distance downstream.

How cool!  Here you can view results of both Tertiary volcanics and Tertiary extension, for Basin and Range tectonics are featured as well.  A short distance south is a normal fault along which the northern block, containing the campground, has dropped.  The resultant deformation is easy to see at the upstream end of the campground.

Sevier River Formation, dipping north on left, nearly horizontal on right.

In this photo looking east across the drainage from the slope above Site 23, the red arrow points to Sevier River beds tilted very steeply, and appearing to lean against the Joe Lott tuff.  At the blue arrow to the north, beds dip much less (white ash layer is a good marker), and they become nearly horizontal downstream.

Here are the same "leaning" slabs (red arrow above) but viewed from below.  This area can be accessed via a trail from Site 17, marked "No ATVs".

The Sevier River Formation is not as photogenic as the more richly-colored sedimentary rocks of the Colorado Plateau; the hoodoos are a bit washed out in comparison, but the patterns are striking, especially in the shady alcoves.

Right:  vertical drainages (slots) develop on Sevier River outcrops, from several inches across to wide enough to enter.  Do hoodoos come about because of this erosional tendency?

The back walls of the slots have beautiful horizontal ripples (below, dog for scale).

Left:  in an especially shady alcove there is a bit more color and warmth in photos ... thanks to the flash.

Below:  the artistic handiwork of erosion.

The Sevier hoodoos and Joe Lott tuff at Castle Rock Campground are featured in Vignette 12 in Geology Underfoot in Southern Utah (Orndorff, Wieder and Futey, 2006), and in the Utah Geologic Survey’s Geosights.  Both sources mention Site 23 as an excellent location to view rocks and structures of interest.  From the west side of the campground loop, informal trails lead into the hoodoo neighborhoods above.

Below:  both the Sevier River Formation (right) and Joe Lott tuff (left) are visible at Site 23. Here at the contact they dip to the north, due to a fault a short distance upstream.

How to get there:

Castle Rock Campground is just south of Interstate 70, about 20 miles west of Richfield, Utah, and 17 miles east of the junction with Interstate 15.  Fremont Indian State Park, close by on the other side of the highway, is worth a visit as well.  Take exit 17 and go south 1.2 miles to the campground.  Sites are $13 per night (2012), and include water, dumpsters, bathrooms, a creek, plenty of shade, a hiking trail up Joe Lott Creek, entry to the FISP Museum, and of course great entertainment for geo-geeks.  Famous Site 23 is on the west side of the loop at the upstream end of the campground ... but be prepared for hordes of geo-pilgrims ;-)