Can someone describe the contact points along the walk - where one could peel out before the end?

Hi Dave,

Hope you and your family are well, hope to see you at the barbaQ. 

There are a bunch of places to peel out early on the walk and they are all pretty easy to determine.  With Research Park and The University Hospital directly below you many access points have been established.  I go up there all the time since I live so close.  Where they are probably comming out is just next to the JCC another place of access.

Cheers,

Gleed

Thanks, Gleed!

Can't wait to see you and the others who can come too!

-Dave

Bri (and Tom Wood), thanks for the fun and fascinating shoreline walk. A dumb question that I've never understood came back to me while I was reading your handout and looking at the last figure that shows all of the different rock layers visible in the Wasatch Range near SLC with their corresponding geological eras (Proterozoic, Cambrian ... Tertiary, Quarternary). The diagram seems to indicate that the oldest rock, 3 billion years old in some cases, is at the bottom and that rock piled on top of that since so that exposed layers correspond to older and older layers. My question is: when did laying down of rock start and exposing of rock begin?

For instance, the layers of the Tertiary were laid down 24-31 m.y. ago. Was the plate underwater until the end of that era? Can rock be "laid down" any way other than when under water?

And the Quarternary valley fill is the erosion residue from the basin broadening, so the continent was above water then?

And how does the answer to this question relate to what Tom said about the 20,000 foot above sea level dome that has been eroding for a long time? How long did he say?

Here it is 5 years after the reunion, and I only recently saw Dave’s questions about geology of the Wasatch Front that he posted after the reunion hike. Tom “Wood” Clawson (Dave, you missed another name), who added a lot of commentary on the hike, also ran across the questions recently, and we decided to prepare a combined answer for you, just 5 years late.

The best place to find out the history of periods of deposition and erosion around Utah is in “Geologic History of Utah,” a small book by Lehi F. Hintze, BYU Geology Studies, Vol. 20, Pt. 3. It could be out of print, just like most of the books we used in college, but the Utah Geological Survey on West North Temple might carry it. It is based on the work of hundreds of geologists, with 18 pages of references to published studies in the literature (what a concept, wink, wink). Hintze, who was at BYU, and William Lee Stokes, who taught at the U of U, were the grandfathers of unifying our knowledge of Utah Geology. When Kris and I returned to Utah from Boston in 1976, I worked on a graduate degree at the U, and I was able to TA for Stokes’ “Stratigraphy of the Western Interior.” I enjoyed many talks with him, and sat in on all the classes to learn about Utah geology.

Anyway, you asked about when there were periods of deposition and when there were periods of erosion. Geologists can tell from the contact between one geologic formation and the next whether the boundary represents continuous deposition over time or whether there was a break in the deposition, with or without with a period of erosion. I have pasted in the “geologic section” representing the rocks in the Wasatch Mountains near Salt Lake that I handed out on the hike, but I am sure you saved your copy, right?

If you look at the section carefully, the boundaries between the geologic strata are either straight or squiggly horizontal lines. Straight lines mean there was continuous deposition from one strata to the next. The change to the old to the younger formation was due to a change in the depositional environment. This would be like changing from marine (ocean) to shoreline or marine to delta, or lake to river, etc. Squiggly lines represent periods of non-deposition with some erosion of the older formation. From the number of squiggly lines, you can see there were a lot of periods of non-deposition from the start of deposition of the oldest Pre-Cambrian rocks through the deposition of the youngest deposits, the valley fill.

(None of the figures would copy in. If you would like a pdf of the whole response with the figures, let me (Bri) know by email and I'll send it to you.)

I have also included a figure from Hintze that gives the arm-waving explanation of the major geologic events in Utah. The whole area has been part of one plate and all of this detail happened as that plate moved and interacted with other plates.

• Phase I: From Late Precambrian through Devonian, the deposits were mostly marine (in the miogeocline, don’t ask what that is). The craton is the continental core of older, Precambrian rock.
• Phase II: From Mississippian through Early Triassic the deposits were also mostly marine, with the exception of some sandstone that was wind blown sand. The Uncompahgre uplift was above water.
• Phase III: Late Triassic to Early Cenozoic, Age of the Dinosaurs with mostly continental deposits, many marshes, rivers, deltas, a little marine. Mountains to the west, and notice that the “gap” from earlier times closed.
• Phase IV: Latest Cretaceous to Eocene. This is when the area was uplifted and there has not been any marine deposition since then. We got the Uinta Mountains, the San Rafael Swell, the Monument Uplift, etc.
• Phase V: Oligocene is when there was a lot of volcanic activity. You see this in southwestern and central Utah with the black volcanic rocks.
• Phase VI: Miocene to Recent is when we had the faulting of the basin and range, tilted blocks and erosion to fill in the basins. Tom is going to comment on the dome that has been eroding.

You asked if rock can be laid down in any other way than by water. Yes, it can be laid down by wind, and sometimes by “mass movement” like landslides. You asked if the land was above water during the valley fill period. Yes, it was after the Late Cretaceous.

This is Hintze’s table that goes along with the phases:

So hopefully that answers some of your questions. Tom has prepared an answer to your final question. As I have hiked with Tom, I realized that when he worked for Exxon (before law school), they had him study much of what he is talking about. He knows the Wasatch very well.

Hi Dave—You wondered how long the large continental uplift (or dome) that stretched from the Wasatch Front to what is now the Sierra Nevada has been eroding? That uplift may have risen as high as 20,000 feet above sea level at some time in the past. The dome, which is a thickening of the crust caused by a sequence of mountain building episodes, initially started rising during the end of Hintze’s Phase II in the early Triassic (200(?) mya), and erosion began as soon as the rocks rose above sea level.

The initial uplift was associated with an island arc—a chain of volcanoes like present-day Japan—that formed above a slab of subducting oceanic crust beneath what is now western Nevada. (The Sierra Nevada represent the roots of that arc.) Eventually, the arc collided with the North American Plate, which caused a great deal of compression inland. The compression forced sheets of crustal sediments (there was little basement involvement) to shift eastward up to ti0 miles on enormous flat-lying thrust faults. The thrusted sheets were 10’s of thousands of feet thick, but mostly remained below sea level during the thrusting, except where they ramped up toward the surface in the vicinity of the Wasatch Front. (These thrusts were facilitated (in part) by high-pressure alkaline water at depth.) This is the event that created the prevalent folds in the rocks that are exposed in the Wasatch Mountains between Mt. Olympus and the Avenues.

I don’t know how high the mountains were that were created by the thrusting, but thick sections of the debris that was shed off the mountains are exposed in many places in the Wasatch. The rocks in the Red Narrows secton of Spanish Fork Canyon and in Echo Canyon on I-80 east of Coalville are good examples. In some places in the foreland basin the sediments reach thicknesses of 20,000 feet. This mountain-building event is known as the Sevier Orogeny, which lasted from about 105 mya to 65.5 mya. The plateau east of the current Andes Mountains in South America is similar to what was happening in Nevada and western Utah during this time. We’ve attached a diagrammatic sequence of cross-sections (the vertical strip) that illustrates these concepts.

During the early Cenozoic, the interaction between the North American and Pacific Plates changed and what had been a normal volcanic arc above a steeply-dipping subducting slab became a shallow, gently-dipping subducting slab of oceanic crust. The slab basically moved horizontally beneath the overthrust sheets in Nevada and Utah and may have moved as far east as the Rocky Mountains in Colorado. Instead of generating volcanoes—because the subducting slab was only partially melted—different types of volcanic rocks (depending on the location) were intruded and extruded. This is diagrammatically depicted on the second (horizontal) figure. There was also a great deal of uplift along steeply-dipping reverse faults associated with this event. It’s called the Laramide Orogeny and lasted from about 65.5 mya to about 50 mya. The very complex thermal regime beneath what is now Nevada continued to uplift (and ultimately weaken) the crust. This probably was the time of maximum uplift.

Things really changed during Hintze’s Phase V (around 30 mya) when the North American Plate intercepted the Pacific Plate’s spreading center near the Mexico-Southern California border. Subduction stopped along the coast and the San Andreas Fault was formed. This relaxed the compression under Nevada and western Utah and the crust responded by expanding (and thinning). The expansion caused the dome to collapse. Large blocks of crust dropped down along normal faults (such as the Wasatch Fault), leaving the highlands where they were. Valleys formed between the mountains and erosion began in earnest to fill the valleys with sediment. This is the origin of the Basin and Range. Some began in earnest to fill the valleys with sediment. This is the origin of the Basin and Range. Some estimates of the expansion surmise that the Basin and Range has doubled in width. (Reno used to be a lot closer to Salt Lake.)

By calculating the volume of sediments in the valleys, if I remember correctly, Dr. Stokes estimated the volume of rock that must have been eroded from the highlands to fill the valleys. These estimates are the basis for his conclusion that the dome could have been 20,000 feet above sea level. Your question, however, caused me to revisit Stokes’ estimate and I realized that at the time he made the sediment calculation (based on geometry), he would not have had the benefit of some new information about the nature of the expansion of the Great Basin that became available in the 80’s as revealed by seismic studies. (Basically, we now know that the normal faults, which have caused the expansion, flatten with depth, changing the geometric assumptions.) Therefore, his 20,000 foot estimate probably was not accurate. In any case, we know that the dome was elevated because some of the remnants of the dome along the Wasatch Front are still above 11,000 and 12,000 feet (making for some good skiing).

So, to answer your question, erosion of the highlands in the Great Basin has been going on for a very long time (200 my?), although the greatest erosion probably has occurred in the last 25 my. I recognize that this is a lot more answer to your question than you probably were interested in, but I think the geological evolution of the western US is amazing. I get excited about what I’ve learned (albeit 30 years ago) and, like Robert, can’t help myself and have to do my arm-waving and sharing.

Bri and Tom: Fascinating! Thank you so much!

Just in time for the 45th. You seem to have been the 1057th viewer of my question!

I sure seem to have trouble with those Clawson names! (Thanks, Tom, for overcoming the offense :)

Absolutely I want the PDF! (david_busath@byu.edu)

Orogenic: ore-generating period?

Another dumb question, following from that one: From physics we learn that it takes very high energy to form heavy elements, like the heavy metals, from lighter elements, like carbon, nitrogen or oxygen. Ore (copper, silver, gold) on the surface: did it develop during extreme pressure periods (at depth) after the formation of the planet? Or are the veins randomly found throughout the crust and developed instead in our solar system's predecessor star before it went super nova? If the geologists don't know, are there any cosmologists out there?

More dumb questions to follow after I study the pdf a bit.

-Dave

Hi Dave--"Orogenic" simply refers to a regional mountain-building event. It's during such events, however, that ore bodies are concentrated. It's my understanding that the heavy elements would have been formed during our sun's predecessor'supernova. They've been part of the earth since it formed (6,000 ya?). The ores are associated with igneous intrusions and emplaced in the crust near the surface as part of the hot aqueous solutions (there's the saline water again) that contain the gold and silver, etc. The veins of valuable minerals are formed during the intrusive process (as it cools). The elements are generally disseminated in the crust/mantle otherwise.

Tom

Remembering the time when our parents were called into meet with teachers because the

cousins Tom and Bri and I were racing to complete our schoolwork before everyone else in the

class.  It was decided that the cousins would be separated into different classrooms and that I be

moved up a grade-- which my mother nixed due to the fact that I have an August birthday and

needed to be oldest, not youngest in my grade (for many reasons!)

The way Tom and Bri have developed into such outstanding scholars/ experts in their

professions is a real credit to our class.

My and Sr Class President Eric Hayes' election to "The Car Salesman's Hall of Fame" may seem

to pale in significance, but remember it takes everyone to make the world work~~!!

I remember that well. Tom and I were never together from 4th grade on. I think we are over some of the competition. However, I think you, Richard, were the one who was usually done first.