Days 4 and 5: Salt Lake City, Utah
Of all the places for my car to start hemorrhaging power steering fluid, Salt Lake City turned out to be one of the better ones. I dropped it off at the mechanic and then my cousin Scott distracted me with a trip up Little Cottonwood Canyon to one of his all-time favorite places – the Snowbird ski resort.
Scott said he always enjoyed introducing out-of-staters to his hometown, but I hope that my constant stream of “oh my GOSH WOW” coming from the backseat on the way up the canyon didn’t get too annoying. I mean, what’s a geologist to do? We passed glacial moraines AND fault scarps AND giant granite intrusions AND hanging glacial valleys AND massive thrust-faulted hodgepodges of sedimentary rock AND not to mention that view of Salt Lake to the west…
All this is possible because Salt Lake City and the adjacent Wasatch Range are perched on a unique boundary – the very eastern edge of the Basin and Range Province of the USA. Yep, you guessed it, it involves the Laramide Orogeny like everything covered in my last two posts, but also the Laramide’s fraternal twin mountain-building event. Meet the Sevier Orogeny.
Both the Sevier and the Laramide happened at roughly the same time (70-50 million years ago) on account of the same pressure (the subducting Farallon Plate). However, two areas of the USA responded differently to the pressure. In areas further east, such as Colorado, eastern Wyoming, and Montana, that pressure hit areas of the continental basement which had been weakened when the supercontinent Rodinia was ripped apart 750 million years ago (mya) and the Ancestral Rockies rose around 300 mya. The weakened continental basement rock buckled under the stress. Geologists refer to this as “Laramide-Style” orogeny, and I saw its results in the Colorado Rockies and the “basement-cored” ranges in the South Wyoming such as the Rawlins and Rock Springs uplifts.
The pressure from the colliding and subducting plate manifested differently further west (Utah, Western Wyoming and Montana) where the continental basement rocks had not been cracked by previous mountain-building or continental rifting. Here, the many layers of sedimentary rock deposited in the Cretaceous Seaway took the strain as the basement rocks got scrunched together. These thin layers cracked and thrust over each other like shuffled decks of cards, creating the thin-skinned “Sevier-Style” orogeny. This style is evident in jumbled, repeated bands of rock in the Wasatch range. The corresponding geologic map looks like one of those scribble-and-fill masterpieces that happened when I first discover MS Paint in 6th grade.
The Western USA breathed a sigh of relief once the Farallon plate completely disappeared under the North American Plate around 50 million years ago. The continental basement, full of north-south trending cracks and pent-up tension from the insistent force of the collison, relaxed westward and flexed downward along those lines of weakness. A simplified version of that is shown in the diagram below…
This had some peculiar consequences for Utah, Nevada, and bits of the surrounding states. You can see this from space!
The decompression of the earth’s crust caused a maze of roughly north-south trending valleys and mountain ranges. Additionally, it dropped this whole area to a level where water could not get over the Sierra Nevadas to the Pacific or the Continental Divide to the Atlantic. The Basin and Range Province became a giant version of the Great Divide Basin where water can only flow into its local valley and evaporate, and the Great Salt Lake is the poster child.
The formation of the Basin and Range landscape isn’t anywhere near done, to the dismay of city planners in Salt Lake City. Utah’s capitol sits right on top of the fault zone where the Great Salt Lake’s basin is sporadically sliding down the edge of the Wasatch Range. This is evident along the edge of the mountains where you can see (geologically) recent fault scarps from the highway.
In the Little Cottonwood Canyon part of the Wasatch Range, to add insult to injury, a giant blob of magma rose from the tail of the subducting plate 30 million years ago and punched through the already disheveled layers of sedimentary rock. Most locals refer to the rock as the “white” or “Temple” granite, but the smooth, bare cliffs are actually made of a relative of granite called quartz monzonite that has less quartz and a more even balance of two kinds of feldspar minerals. This massive batholith (geology-ese for “giant blob of magma), now unearthed by millions of years of erosion, is currently home to some world-class rock climbing routes and a Church of the Latter Day Saints top-secret genealogy bunker.
On the way back down the valley it was easy to see traces of the latest force of nature in the canyon. During the last glacial maximum 15,000 years ago, the road we drove on would have been under hundreds of feet of ice! Both Big and Little Cottonwood canyons were occupied by huge glaciers fed by precipitation fueled by the ancient Lake Bonneville, driven up the mountains by western winds, and dumped in the Wasatch Range as snow. As these well-fed rivers of ice scraped downhill they carved out the dramatic steep-walled valley that we see today. The piles of pulverized rock shoved ahead and to the sides of the glaciers remain at the mouths of the canyons and are now mined as construction fill. The same climate pattern bears out today, with a warmer average temperature and a smaller lake, as the powdery snow that Scott loves to shred down at Snowbird.
On Thursday, with my car still up in the air, Scott took my dad and me downtown to see the famous monument built out of Little Cottonwood Canyon’s quartz monzonite – The Church of the Latter Day Saint’s Temple Square.
When the congregation outgrew the original temple they moved to a giant structure that could hold 20,000 Saints at a time and has a forest on the roof! In order to avoid the weight of organic soils up there, the engineers used ground-up shale from the Wasatch Range to anchor the plants and then pile on the fertilizer.
Just as we were leaving the conference center I got the call saying that my car was ready to roll again. That afternoon we said goodbye to Scott and Salt Lake City, and headed north to a very different landscape indeed. Goodbye mountains, hello giant lakes of (cooled) lava!
Stay tuned for Road Trip Part 4: Snake River Plain and Columbia Gorge.