The Seashell on the Mountaintop, a book by Alan Cutler, is the story of 17th-century Danish scientist Nicolaus Steno who effectively discovered “deep time,” the idea that the earth is vastly more than 6000 years old. Steno is rightly considered one of the fathers of geology. Steno’s seashells were high up in the Alps, but similar situations abound, from Mt. Everest to Mt. Nebo in Utah.
At about 3,637 meters (11,933 feet), Mount Nebo is the highest point in the Wasatch Range of Utah, USA. It isn’t the highest point in Utah, but because of its relative isolation along the mountain front and 1,500 m (4,900 ft) of topographic prominence, it is one of the most dramatic in the state.
Mt. Nebo consists mostly of a thick sequence of rocks, mostly limestones, from the Oquirrh Formation, or more properly the Oquirrh Group, deposited mostly during the Pennsylvanian Period (Late Carboniferous), but perhaps extending down into the Mississippian (Lower Carboniferous) and up into the Permian, spanning the time from about 320 to 280 million years ago.
The Oquirrh Group is exceptionally thick, at least 15,000 feet (4600 m) and possibly as much as 24,000 feet (7000 + meters). It’s challenging to explain such thicknesses, but the best explanation is related to far-flung interactions on the eastern and southern margin of North America. At the time the Oquirrh Basin was forming, North America was colliding with Gondwana (Africa + South America and more) as part of the assembly of the supercontinent Pangea.
Like the modern continent-continent collision between India and Eurasia, a huge Himalaya-style mountain range developed along the collision zone, eroded today to the Appalachians, Ouachita Mountains in Arkansas, and Marathon Mountains in West Texas. But there were intense reactions far from the collision zone.
Western North America buckled, producing a suite of complex uplifts and downwarps called the Ancestral Rockies. The rapidly subsiding Oquirrh Basin is part of that, but probably complicated by some major strike-slip faulting within or adjacent to the western margin of North America, which was approximately in western Nevada at that time (Jones and others, 2021, Sediment routing and provenance of shallow to deep marine sandstones in the late Paleozoic Oquirrh Basin, Utah: Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 578, 15 Sept. 2021, 110582). The effects were similar to the deep troughs forming today in the California Borderland, where the Santa Barbara Channel holds more than 6000 meters (20,000 feet) of sediment deposited in just the past six million years or so, but on a smaller scale than the Pennsylvanian Oquirrh Basin (Sorlien and others, 2000, Map restoration of folded and faulted late Cenozoic strata across the Oak Ridge fault, onshore and offshore Ventura basin, California: Geological Society of America Bulletin 112: 1080-1090).
Limestone in arid country like Utah is resistant, forming prominent ridges and mountains like Mt. Nebo. But the height and prominence of Mt. Nebo are related more to tectonics than the nature of the rock.
About 200 million years after the Oquirrh rocks were deposited, western North America began its own collisions with various outboard terranes, resulting in subduction zones and pushing rocks from the west to the east. In central Utah, the western crust, with a thick cover of sedimentary rocks including the huge pile of Oquirrh limestones, traveled eastward on thrust faults until the package reached the thick strong buttress of the rocks in today’s Colorado Plateau. This is called the Laramide Orogeny, and with considerable variation is the origin of the cores of many of the high mountains of Colorado, Wyoming, and Montana about 80 to 50 million years ago.
Once the collisions ended, compressive stress and pushing ceased. A degree of relaxation began, aggressively perhaps 35 million years ago and especially around 15 million years ago and continuing to this day. Once again the Colorado Plateau stood thick and strong while the weaker rocks to the west broke into what we now call the Basin and Range in western Utah and most of Nevada.
The bounding edge between the strong Colorado Plateau and weak Basin and Range is the prominent, high Wasatch Range through central Utah, with Mount Nebo sitting on the east side of the boundary (the Colorado Plateau side) where its rocks were thrust in the Laramide Orogeny. The prominence of the preserved Oquirrh Limestones in Mt. Nebo owes to the Wasatch Fault System, just west of Mt. Nebo, with much of Interstate 15 following along the low, downdropped side.
I did some oil exploration work in this area for Gulf Oil back in the early 1980s. The focus was on possible oil-bearing rocks, such as fractured or porous Oquirrh Limestone, that was buried and had proper geometry to trap oil and gas. I used gravity and magnetic data to try to define structures. I think the biggest problem with the play was the lack (or complications) of adequate source rock.
Although there are lots of marine invertebrate fossils in the Oquirrh rocks, including corals, brachiopods, and more, some of the most abundant are fusulinids, single-celled animals that secreted calcium carbonate to make shells. Many were large for single-celled animals, up to 5 cm or more in length. The word fusulinid is from Latin for “spindle,” for their shape.
“Oquirrh” is from a Goshute or Ute Indian word meaning “wooded mountain” or “shining mountain.” Mt. Nebo was named by Mormon settlers after the Biblical mountain where the Old Testament says Moses died.
What a lovely write-up Richard. I really enjoyed this. You have a gift with writing weaving your past work with others and all these neat maps and photographs. It's very clear and easy to understand but not too dumbed down either, I appreciate some of the technical info. How you bring Mnt Nebo into geologic context is just the best reading. I really am a sucker for those U.S. paleomaps too. It all makes my head explode in a good way. I'm new to Substack. I love the idea. I really hope it catches on because content like yours IS the reason that it should suceed as I totally believe if more people could try Substack they would fall in love with authors such as you, I did. Good job and thanks for putting this out there for curious arm chair scientists to further geek out at the world and hopefully pass the torch on to my kids. Cheers!
Thanks for this article and lovely illustrations, including the reflective last photograph. I enjoy your posts and learn a lot from your writing. Thank you for all the time and effort you put into creating them.