Devil's Playground—another intrusion, this one with wildflowers

Late afternoon at the Devil's Playground in northwest Utah—my kind of #vanlife.

I ended my tour of Utah and Nevada last May with a stop at the Devil's Playground north of UT Highway 30 near ... well ... near not much of anything. It's at the south end of the Grouse Creek Mountains and about 40 air miles west of the north end of the Great Salt Lake. The Utah Geological Survey provides directions here.

Like several other stops on the trip, this one featured an igneous intrusion—the Emigrant Pass pluton, emplaced 41 to 34 million years ago in three phases. Rock in the Devil's Playground area is part of the youngest phase (Egger et al. 2003). This pluton is especially interesting to geologists studying metamorphic core complexes (MCCs), for it is in the southern part of the Albion–Raft River–Grouse Creek MCC (ARG in map below).

Black blobs are MCCs; arrow points to Emigrant Pass pluton. After Strickland et al. 2011.

Note multiple plutons in Albion, Raft River, Grouse Creek Mts. After Egger et al. 2003.

The diagram below shows a textbook metamorphic core complex (actually there may be too much variation to have a classic example). During continental extension, rock strata stretched, broke, and slid along low-angle faults, revealing older deeper rocks which domed upward. But continental extension has occurred across much of the Basin and Range Province without making MCCs ... hmmm, puzzling.

Based on Peterson & Buddington 2014, DeCourten & Biggar 2017.

As I learned during my trip to the Ruby Mountains, MCCs are controversial. Among topics debated is why they're clustered in this part of North America (thickened crust?). Another is the role of plutons in MCC formation. Some geologists argue that plutonism is the main driver, supplying heat that softens rocks and facilitates extension. After all, plutons roughly contemporaneous with extension "are ubiquitous in many of the core complexes" in this region. But other geologists disagree, arguing that plutonism plays a minor role at most (see Introduction in Egger et al. 2003 for more discussion).

I like metamorphic core complexes very much, largely for their mystery. But they're difficult. It's challenging just to spot one, even with a guidebook. These are giant structures, visible only as parts exposed here and there. Plutons are much easier to understand, fairly common, and yet still worth contemplating. I became a fan when I realized that if I can see a pluton, something dramatic must have happened.

It seems plutons are often sculpted into intriguing forms, like the Harrison Pass pluton in the Ruby Mountains.

Tors carved from one of our local plutons, at the crest of the southern Laramie Mountains.

Notch Peak intrusive at the base of the west face of the House Range; photo by Mike Nelson.

Emigrant Pass pluton—a tilted world. Did it tilt during uplift? 

Plutons, like the god Pluto, reside in the Underworld. They form when magma solidifies well below the surface, where it cools slowly and forms visible crystals. So unless we make a Dante-esque excursion miles underground, we can only see plutons if they've been exposed in some way. When the Grouse Creek Mountains rose about 13 million years ago, during Basin and Range extension and faulting (Ege 2006), erosion set in. That's probably when the "devils" of the Playground were born.

Rocks in this part of the Emigrant Pass pluton have been called granitic and granitoid. These are handy terms because the range of granitic rock types is broad and hard to subdivide neatly. Egger et al. (2003) are more specific: "a virtually homogeneous coarse-grained biotite granite". The granite is criss-crossed with aplite and pegmatite dikes, which formed when still-molten magma—hydrous and therefore last to crystallize—was injected into fractures in the solidifying pluton.

Aplite is more resistant to erosion so dikes stick out a bit from the granite.

My field assistant pointed out a dike in a different kind of granitic rock.

Weathering of the pluton may have started underground, with groundwater enlarging fractures. In any case, today's wonderfully enigmatic forms are largely products of physical and chemical weathering above ground (Ege 2006 explains this nicely). These processes will continue until eventually the alcoves, spires, arches, fins, and other devils disappear.

Fine example of spheroidal or onion-skin weathering at the Devil's Playground. Photo courtesy scienceteacherexplorer (click link for more great shots).

Geocacher enjoying spheroidal weathering (source).

Are these young devils, recently emerged? Or elderly ones, to dust returning?

In the company of plants and rocks :)

Among the devils were spring wildflowers, a nice touch. Perhaps the most common was Stenotus acaulis, the Stemless Mock-goldenweed. It grows on rocky soils in drier areas across the western US. This is a DYC—"damn yellow composite"—but only because we find yellow composites difficult to identify. Seems to me that's our damn problem.

Stenotus acaulis; those of us who have been around for awhile may know it as Haplopappus acaulis.

Another yellow composite (Compositae is the old name for the sunflower family) caught me by surprise—Balsamorhiza hookeri, Hooker's Balsamroot, a plant of the Great Basin. I know Arrowleaf Balsamroot well, but didn't recognize this plant as a balsamroot. Based on online specimens and discussions, the plants here might be hybrids; more research needed.

Hooker's Balsamroot seems so different from Arrowleaf Balsamroot. Most strikingly, it is short (these plants are 10 to 15 cm tall), and can thrive on very dry rocky sites.

With so much sagebrush in the area, it wasn't surprising to find its common parasite—paintbrush; this one is Castilleja angustifolia, the Northwest Paintbrush. The low gray-green shrub next to it in the photo is sagebrush. Like DYCs, paintbrushes are difficult to identify to species. But I've never heard anyone damn them. Thanks to markegger for the identification, via iNaturalist.

Paintbrushes are hemiparasitic. They can photosynthesize, but by tapping into sagebrush roots they grow more vigorously. This ability may vary among species, perhaps explaining conflicting reports online.

Such a lovely parasite!

Sources

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press.

Ege, Carl. 2006. Geosights: Devil's Playground, Boxelder County, Utah. Utah Geological Survey Survey Notes 38 no. 1, January 2006.

Egger, AE, et al.  2003. Timing and nature of Tertiary plutonism and extension in the Grouse Creek Mountains, Utah. International Geology Review, 45:6, 497-532. https://doi.org/10.2747/0020-6814.45.6.497

Peterson, J, and Buddington, A. 2014. A geological study of the McKenzie Conservation Area, Spokane County, Washington. Conference Paper.

Strickland, A, et al. 2011. Timing of Tertiary metamorphism and deformation in the Albion–Raft River–Grouse Creek metamorphic core complex, Utah and Idaho. J. Geol. 119:185–206. https://doi.org/10.1086/658294

House Range: more than a Tertiary fault block

Sun sets on the House Range, west central Utah. Thanks to Mike Nelson for photo and info.

Last month I wrote about my stay in the House Range, a large mass of rock that started as sediments in a Cambrian sea. Now 500 million years later, they stand nearly 10,000 ft above sea level. On the steep west face the layers are obvious. On the opposite side we see gentle slopes that disappear below the surface of the valley to the east.

Steep west face, with the "well-defined stratification" observed by Capt. JH Simpson in 1859. Because the crest looked like structures, he called these mountains the House Range.

Road leading to the east side of the House Range. Note the gentle slopes. The pale rock in the draw on the right is not sedimentary.

Pioneering geologist Grove Karl Gilbert visited the House Range in the early 1870s, as part of GM Wheeler's Surveys West of the One Hundredth Meridian. The asymmetry of the uplift blew his mind. "... the beds exhibit in cross-section but a single direction of dip" (italics mine). He had expected something like the Appalachian ranges, whose structure was considered typical of all mountains. "... it was only with the accumulation of difficulties that I reluctantly abandoned the idea." (Gilbert 1875)

Gilbert's cross-section through the House Range (1875).

Of course surprises were to be expected; geology was still a young science. For example consider orology—the science of mountains (now orogenesis). In Gilbert's time, geologists struggled to explain how mountains formed even in a very general way. Their theories ranged from crustal wrinkling as the Earth cooled to fiery forces underground.

Gilbert admitted he was far from understanding orology in the "Basin Range System" (now Basin and Range Province). But after studying so many ranges he could shed some light on the subject. He suspected the House Range was bounded on the west by a steep normal fault, which had tilted strata downward to the east. And whatever caused this uplift must have been operating on a grand scale, for he had seen similar structures over a huge area.

The Basin and Range Province overlaps the Great Basin but is larger, mainly to the south (sources differ on boundaries). In the BRP, ranges generally trend northerly, are bounded by normal or listric faults, and are separated by sediment-filled basins.

Gilbert also discovered "cross faults" in the House Range. He included one in his diagram of the west face (below), which emphasizes the southerly component of overall dip. In several places, strata have been disrupted by faulting. Gilbert shows this by labeling beds of quartzite ("q", looks like "g") and limestone ("l"). Near Notch Peak the quartzite and limestone are higher than they are at Dry Pass to the north, contrary to what we would expect based on dip. Gilbert had no explanation for these faults, except that they probably predated uplift of the House Range (Hintze & Davis 2003).

Click on image to view displaced quartzite and limestone (Gilbert 1928).

Today geologists generally agree that Basin and Range orogenesis is due to stretching of the continent, as the Pacific and North American plates grind past each other along transform faults (DeCourten & Biggar 2017; see Busby's Walker Lane diagram). Extension is thought to have started 30–40 million years ago, and so far has doubled the distance between Salt Lake City and Reno. It continues, as evidenced by earthquakes and precision GPS measurements. As Gilbert suspected, this is orogenesis on a grand scale—from eastern California to central Utah, and from southern Oregon and Idaho to northern Mexico.

Gilbert also was right about the relative age of the cross faults in the House Range (Hintze & Davis 2003). They were part of earlier mountain building (late Jurassic through Cretaceous), when western North America was being compressed as the Farallon plate dove under the west coast. The result was 200+ million years of orogenesis, producing the Sierra Nevada, Rocky Mountains, and the lesser known Sevier orogenic belt (DeCourten & Biggar 2017).

During the Sevier Orogeny strata were shoved eastward, sometimes great distances, along low angle thrust faults (detachments). Seismic exploration has shown that the central House Range is underlain by several shallow-dipping major faults formed by regional, easterly-directed thrusting, most likely during the Sevier Orogeny (Stoeser et al. 1990).

Non-sedimentary rocks near the base of the House Range's west face.

Oddly, Gilbert seems to have ignored a prominent geologic feature of the House Range, though he may have hinted at it in one sentence: "The rocks are almost wholly sedimentary" (italics mine) (Gilbert 1928, p. 74). The topographic map (Plate 31) provides another hint:

Note "Granite Canyon" northeast of Notch Peak. It's now called Miller Canyon.

Finally, Gilbert's diagram of the west face shows steeply tilted strata below and just north of Notch Peak, but with no explanation (below; red annotations mine).

Are the lower rocks in this photo Gilbert's steeply tilted strata?

In 1905, the eminent paleontologist Charles Doolittle Walcott came to the "great House Range" to study its fossil-rich rocks, and to establish "the interrelations of the strata and faunas in the North American Cordilleran area". In his 1908 report, he included photographs of exceptional sites so that "geologists and paleontologists who have not had an opportunity to see the sections may get an idea of the completeness of the exposures of the strata in the Cordilleran area." One such site was the House Range.

Notch Peak and the west face of the House Range. "... an intrusive mass of granite porphyry is intruded into the Cambrian beds on the north side of the peak (left side)." Aside from this caption, Walcott made no mention of the intrusion in his report.

Being a huge fan of GK Gilbert—such an observant open-minded adventurous geologist!—I have to wonder why he omitted this obvious granitic intrusion in his reports on the House Range. Maybe he just didn't want to struggle with yet another puzzling geologic feature.

But now much of the puzzle has been solved. "The Notch Peak intrusive presents a case study of the whole gamut of magma emplacement ... Seldom can one find in so neat an area the geologic record of such a variety of processes generated by a single intrusive body," wrote Arthur L. Crawford, Director of the Utah Geological and Mineralogical Survey in 1958. No longer do geologists ignore it (e.g. Gehman 1958, Stoeser 1990, DeCourten 2003).

Notch Peak intrusive viewed from east. Intrusions by definition form underground. If we surface creatures can see them, something must have happened—in this case, uplift of the House Range and  erosion.

The Notch Peak intrusive is composed of quartz monzonite, sometimes called granite (they differ slightly in composition). It's assumed to be Jurassic in age (dated at 193–143 Ma), is about 3 mi in diameter or 2.5 x 4.5 mi in area, and may be a laccolith. Other features include aplite dikes and sills intruding adjacent sedimentary rocks, zones of pegmatite, crystal-lined cavities, and mineral-rich skarn.

The beauty of skarn!—from Osgood Mountain intrusive (into carbonates), Nevada. Like Notch Peak skarn, it contains tungsten and molybdenum. James St. John via Flickr.

Intrusions have created great wealth in Nevada and Utah, in the form of ores. As molten magma ascends and crystallizes, hot fluids are expelled. These circulate and dissolve surrounding rock. If conditions are right, new minerals precipitate out in sufficient quantities to produce ore, where "one or more valuable substances can be mined at a profit." (Mineral Resources)

If magma intrudes carbonates—limestone or dolomite—it often creates skarn when saline metal-rich fluids alter the host rock to form new minerals. In the House Range, the Notch Peak intrusive had ample opportunity to alter limestone. The resulting skarn contains tungsten and molybdenum in moderate concentrations (Stoeser 1990).

Head of Miller Canyon: "FEDERAL MINING CLAIM ... ABSOLUTELY NO PROSPECTING ALLOWED".

Note

Here's a simple timeline for the House Range. For dates, check this geologic time scale.

Paleozoic Era, Cambrian Period: Marine sediments accumulate to great thickness off the coast of Laurentia.

Later Mesozoic Era: Sevier Orogeny deforms sedimentary rocks in the area of the future House Range. Notch Peak quartz monzonite intruded into sedimentary rocks during Jurassic Period.

Cenozoic Era, Tertiary Period (continuing to today?): Continental extension with block faulting uplifts the House Range. Erosion sets in, eventually exposing the Notch Peak intrusive.

Souces

DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology. U Utah Press.

DeCourten, F. 2022. The Great Basin Seafloor. University of Utah Press. Supplemental Field Guide (PDF) available online.

DeCourten, F, and Biggar, N. 2017. Roadside Geology of Nevada. Mountain Press Publishing Co.

Gehman, Jr, HM. 1958. Notch Peak intrusive, Millard County, Utah. Geology, petrogenesis, and economic deposits. UT Mineralogical & Geological Survey Bulletin 62. PDF

Gilbert, GK. 1875. Report upon the Geology of portions of Nevada, Utah, California, and Arizona, examined in the years 1871 and 1872 in Wheeler, GM. Report upon United States Geographical surveys west of the one hundredth meridian v. 3. Washington [D.C.], G.P.O. BHL.

Gilbert, GK. 1928. Studies of Basin Range structure. USGS Professional Paper 153. PDF

Hintze, LF, and Davis, FD. 2003. Geology of Millard County, Utah. UT Geo. Surv. Bull. 133. PDF

Simpson, JH (US Army). 1876. Report of explorations across the great basin of the territory of Utah for a direct wagon-route from Camp Floyd to Genoa, in Carson Valley, in 1859, by Captain J. H. Simpson ...: Making of America Books, U Michigan.

Stoeser, DB, et al. 1990. Mineral resources of the Notch Peak Wilderness Study Area. US Geological Survey Bulletin 1749. GPO. PDF

Walcott, CD. 1908. Cambrian sections of the Cordilleran area, in Cambrian Geology and Paleontology. Smithsonian Misc. Collections 1910, v. 53 no. 5:167–230. BHL

Is there beauty in the Great Basin?

"I begin to think the Great Basin, like many other great things in this world, a great humbug ... very interesting to the geologist and geographer, but dreadfully wearisome to the traveler, as we can attest." Cornelia Ferris, 1853

In his wonderful book, The Broken Land, geologist Frank DeCourten begins Chapter 1 by asking why and where we find beauty in nature. Why do the "vistas of sagebrush and mahogany-colored mountains" of the Great Basin that he finds so "thrilling" bore or even repel most travelers? Perhaps it is as Thoreau says—there is only "as much beauty visible in the landscape as we are prepared to appreciate ...".

DeCourten goes further, asserting that beauty and its appreciation will increase with a deeper understanding of the landscape. I agree. With each visit to the Great Basin, the landscape becomes more engaging and grand as my "geological enlightenment" grows.

The Great Basin; note the many "caterpillars" lined up southwest to northeast (by Kmusser).

It was a more recent DeCourten book that inspired my latest trip to the Great Basin—Roadside Geology of Nevada, coauthored with Norma Biggar (2017). Actually this was a second try, after a trip two years ago was aborted due to snow and more snow. This time the weather cooperated marvelously, and I spent two weeks in a geological wonderland with much that was beautiful. Just look!

The Devils Gate Limestone, beautifully exposed along Highway 50, formed from sediments deposited 370 million years ago in shallow water just off Laurentia (North America). At that time the continent was smaller, the west coast located in today's Utah. Millions of years later, the horizontal beds were tilted and then revealed by erosion ... lucky for us!

Devils Gate west of Eureka.

About ten miles north of Devils Gate, near Tyrone Gap, I was stopped in my tracks by more tilted beds, these approaching vertical! The rock is silicified conglomerate dating from 300 million years ago—once-horizontal beds of sediment eroded off the Antler highlands (mountains now long gone).

Steeply-tilted conglomerate of the Garden Valley Formation.

Nevada's accreted terranes were a new experience for me—further enlightenment :) These are chunks of crust that rafted in from somewhere and were plastered onto the continent, extending North America west by hundreds of miles. Chalk Mountain (not chalk but dolomite) was carried here on the Paradise terrane, which arrived sometime in the Mesozoic (the dolomite has been dated as late Triassic).

Chalk Mountain (actually dolomite) ended up here from parts unknown. 

On the heels of the Paradise terrane came the Sand Springs terrane, mostly deep water sedimentary rock (shale) that was metamorphosed to slate and phyllite. The platy rocks shone and sparkled in the sunshine, which I enjoyed immensely! But my camera had trouble capturing their darkness. It did better with my souvenir.

Phyllite outcrops just west of Sand Springs Pass on Highway 50.

The Pine Nut terrane is easily accessed via Highway 338—a bit out of the way but worth the trip! At Milepost 6, it squeezes between two large Pine Nut outcrops. There's room to park and not much traffic. These were once volcanic rocks of a marine island arc, metamorphosed during accretion probably. Among my souvenirs is a beautiful rock whose contortions testify to its mysterious but exciting past.

Pine Nut terrane 6 miles east of California.

The rock on the right is my favorite currently.

Many of the outcrops I saw were volcanic, products of huge violent eruptions that took place during an episode of volcanism lasting 25 million years. Viscous magma exploded from giant calderas, covering the land in thousands of feet of ash so hot that it was welded into rock. It was Hell, right here on Earth!

Tuff erupted 28-25 million years ago; now beautifully eroded and exposed in Gabbs Valley Range.

On top of all this craziness, the continent here is undergoing extension. For reasons still debated, the Great Basin is being stretched, and Nevada is now nearly twice as wide as it was 30 million years ago. In the process, basins drop and mountain ranges rise (creating the caterpillars in the Great Basin map above).

This stretching and buckling hasn't stopped. Just 67 years ago, a magnitude 7.2 earthquake boosted Fairview Peak 7–20 vertical ft; the ground moved 3–13 ft horizontally as well. A beautiful fault scarp is clearly visible from the gravel road along the base.

Fairview Peak earthquake fault scarp.

As a summary, I offer this final photo. Ash flow tuff exploded from a caldera 32 million years ago now frames the distant Toiyabe Mountains, where Paleozoic metamorphosed marine sediments intruded by Mesozoic granitics are rising as the land continues to stretch. Yes, there is beauty in Great Basin ... lots of it!

View west across the Big Smoky Valley, from the mouth of Northumberland Canyon.

Sources

DeCourten, F. 2003. The Broken Land; adventures in Great Basin geology.

DeCourten, F. and Biggar, N. 2017. Roadside Geology of Nevada.

Ferris, Cornelia. 1853. From Ferris & Ferris 1856, The Mormons at Home: With Some Incidents of Travel from Missouri to California, 1852-3. In a series of letters.

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 ;-)