Western Wyoming Rocks
As we have traveled around and captured images of some pretty incredible places, the desire to know more about what makes them so fascinating has grown. Many of the iconic destinations in the U.S. are iconic because of the spectacular geologic features. So I decided to brush off that college degree in Geology and understand the story behind those rocks we see.
On a trip last summer, Gayla and I visited friends in the small town of Dubois, Wyoming located 50 miles east of Grand Teton National Park. Our friends, took us for a nice ATV ride back up into the mountains. We stopped along the way to take in the spectacular view of the Wind River Basin and the Absaroka Mountains. The geology is incredible, but to my dismay, I hadn’t done my homework and despite the college degree, I had nothing significant to offer to the discussion. I promised some answers to some simple question; what are we looking at and how did it get that way? Researching the information is easy, the challenge is how to tell the story without putting y’all to sleep! Gayla reminded me not to be “a geek” and she does the editing. I will try.
A friend sent me a video of the geologic history of Colorado. This is a great start and a great resource since we are just “looking over the fence”.
From that video I learned that if we were standing where Wyoming was between 540 and 300 million years ago, we would likely need a couple things, sunscreen and a boat. This is because our journey started south of the equator and moved north. Life in the tropics and often out to sea. The white box on the globe is Colorado, but I figured that’s close enough! 🙂
The globe pictures were from the video in the link above and show the earth 540, 450, 350 and 300 million years ago left to right.
Paleozoic Era (541-252 million years ago)
The rocks being deposited during this time were coastal sandstones and shallow sea deposits like limestones and dolomites. Rock formations you may recognize include the Flathead Sandstone (beach deposits), Gros Ventre Shale, and the shallow sea deposits including the Gallatin Limestone and Bighorn Dolomite. The shoreline fluctuated for many reasons including sea level changes and what was going on with how the continental plates were interacting with each other.
Around 320 million years ago, our neighbors to the south in Colorado would find themselves dealing with the Ancestral Rocky Mountains growing up around them. It wouldn’t have affected Dubois directly since we were still likely on a boat. But, it will play a part later in depositing some of the rocks we see around us. In the meantime, the rocks being deposited are the massive blue-gray Madison limestone and, as the shoreline fluctuated, the Amsden, Tensleep, and Phosphoria formations. Additionally, the continent was expanding to the west as island arcs offshore began attaching themselves to the western coast.
The Ancestral Rockies grew for 20 million years or so and then began to erode. By around 250 million years ago, those mountains had eroded flat and they undoubtedly contributed to the rocks deposited across Wyoming.
Mesozoic Era (252-66 Million Years Ago)
250 million years ago, the major continental plates had become more prominent and joined together to make one large continental marriage called Pangea. But, things were complicated and the marriage only lasted 50 million years or so. Consequently, around 200 million years ago, the plates began to go their separate ways. As the details of the separation were being worked out, Wyoming continued to head north.
Had we been hanging out in Dubois 250, 200 and 150 million years ago as the globes above indicated, we would have seen some major changes. First, we could have parked the boat, at least for a while because the seas receded. Second, the climate would have gone through large changes from being dry, desert-like with large wind-blown sand dunes to humid and what we see in the Everglades today. Hard to dress for all those changes. One signature formation was the Chugwater Formation known for it’s distinctive red color. The Chugwater rests on top of the Dinwoody Limestone and below the tan-colored Nugget Sandstone.
Finally, it would probably have also been a good idea to upgrade the fence because large dinosaurs also roamed the area during the Jurassic Period. The Morrison Formation, deposited during the late Jurassic was a major source of museum quality dinosaur bones with significant fossil-producing outcrops north of Laramie, Wyoming.
The Seas Returned
The overall elevation remained around sea level during this time and once again an inland sea returned and covered most of Wyoming. There was a few million years around 180 million years ago where an inland sea, the Sundance Seaway, moved on in but you might have had beachfront property in Dubois. The Cretaceous Seaway or Western Interior Seaway as it is called rolled in around 100 million years ago, or so, and covered the entire state. That seaway hung around for the next 40 million years. Time to get the boat out again.
Most of the deposits during the Cretaceous Period (145-66 million years ago) associated with the interior seaway were sandstones and shales most having a distinctive gray color included, common for deposits during this period. Names included the Cloverly formation which is a variegated red and white sandstone/shale, Mowry, Thermopolis, Frontier, Cody, Mesa Verde and Meeteetse formations. Many of these layers also included coal and bentonite.
Sevier Orogeny (140-60 million years ago) and Laramide Orogeny (65-50 million years ago) – Fancy name for mountain building events
Well, the divorce from Europe, South America and Africa was complete, Wyoming was no longer heading north and a new, rather dynamic relationship had formed between the North American plate and a continental plate off the west coast called the Farallon Plate. Basically, North America was running over (subducting) the Farallon plate in the collision.
The collision would leave a mark. The earliest mountains to form were those in SW Wyoming often referred to as the “overthrust belt” during the Sevier Orogeny. This is where the collision forced “thin” layers of the crust to break off and start stacking on top of each other. But, as the crash developed, the very old basement rocks started to get squeezed and began to fault upward during the Laramide Orogeny. The Laramide formed the Wind River, Owl Creek and Big Horn Mountain ranges as well as most ranges to the south in Colorado. This was a big event that set the stage for what we see today in the mountain states!
Many of the old layers of rock that had been laid down for the last several hundred million years were now being exposed as they were tilted up or rocks covering them eroded on the tops of the mountain ranges. Very, very old, like over 1 billion years old, were now being exposed in the Wind River, Owl Creek and Big Horn Mountains.
The tilted rocks in the image below are found southeast of Lander. The red Chugwater Formation is easy to spot but below it are the Dinwoody and Phosphoria formations and above it is the Nugget Sandstone representing over 50 million years of deposits, over 200 million years old, brought to the surface by the Laramide Orogeny…an amazing and beautiful sight to see!!
Cenozoic Era (66 million years ago to Present)
What goes up, must come down. As nature has it, the mountains that rose up across Wyoming now start erode. All those layers of rock that were lifted up on top of the mountains are now feeling the effects of weather and gravity. Eventually, most of the sediments that were on top of the Wind River Mountains eroded away leaving the basement granite and metamorphic basement rocks exposed. The sediments eroding off the mountains filled the basins starting new rock layers like the Fort Union, Indian Meadows and the red and white layered Wind River Formation which surrounds Dubois.
Absaroka Volcanics
About 40 million years ago, Dubois and much of the surrounding area would probably seen some significant drops in housing values when volcanoes started moving in the neighborhood. These volcanoes deposited thousands of feet of lava in a large area north of Dubois extending up into Montana over a 15 million year period.
These layers of rock called the Absaroka Volcanic Supergroup which included layers called the Aycross, Tepee Trail, Two Oceans & Langford, and the youngster Wiggins Formation. Some smaller, localized events followed. These layers of rock make up much of the spectacular Absaroka Mountain Range.
If we were hanging out in Dubois 20 million years ago and decided to make a trip west to Grand Teton National Park, we may have been surprised to come up over the pass and find the Tetons weren’t there. That relationship between the North American and Farallon plates had changed from compression to extension and caused a fault to form along the eastern edge of the Tetons. Once again, the mountains grew and nature, using erosion, sculpted the masterpiece!
One little tidbit, geologists have found some rocks on the top of Mt Moran on the west side of the fault that match rocks buried deep below the surface on the east side of the fault. They estimate that this fault had approximately 30’000′ of displacement. Holy cow!!!
“Recent” Yellowstone Volcanic Activity
When we think of Yellowstone to the northwest of Dubois, we often make the association with past volcanic activity and the thermal features associated with the “hotspot” that underlies the park. That volcanic activity is not directly associated with the Absaroka volcanics over 25 million years ago. This hotspot is a pool of magma that Yellowstone sits on has worked its was up Southern Idaho (actually the crust has moved over the magma pool) to the current location. It erupted many times over Idaho in the last 15 million years but only a few times over Yellowstone in the last 2 million years, the last being around 630,000 years ago.
The Sculptor
Geologists refer to the Pleistocene Epoch (2.58 million to 10.7 thousand years ago) as the Ice Ages. Glaciers came and went many times during this period. Glaciers were prevalent throughout the mountains and are largely responsible for the spectacular geologic scenery. Glaciers sculpted the Tetons and within the last 200,000 years, glaciers were thousands of feet thick in the region.
The ice ages ended over 10 thousand years ago and the glaciers melted. What we see today are glaciers formed in the last 5,000 years or so during “mini ice ages” as temperatures have and continue to fluctuate. The “U-shaped” valleys are lasting evidence of those glaciers and can be seen across the mountains of Wyoming and surrounding states.
The rivers continue to carve and sculpt the mountain valleys and basins and make some incredibly cool places to just hang out and enjoy the moment. Some of my favorite places to be are along these rivers.
So, after that long answer, the short answer is…
As we sat on the hill and looked across the valley we were seeing the mountain peaks and Ramshorn sculpted from the Absaroka Volcanics, Wiggins Formation. The valley floor largely composed of the red and white striped Wind River Formation overlying the Indian Meadows Formation, all deposited after the Laramide Orogeny during the Eocene Period (58-33 million years ago).
The tilted rocks on the east side of the Winds are remnants of older rocks uplifted during the orogeny. The walls of the valley we rode into and crossed the creek were a sequence of Paleozoic rocks likely ranging from 550 to 350 million years old. These may have included, oldest to youngest, the Flathead Sandstone, Gros Ventre Shale, Gallatin Limestone, Big Horn Dolomite and possibly up to the Darby and Madison Limestones.
These older rocks lay on top of the 1 billion-plus year old granites and metamorphic rocks that make up the core of the Wind River Mountains and exposed across the top of the range.
Gayla and I thoroughly enjoyed the visit! 🙂
For a deeper dive, here is a link to a USGS website:
Mineral Resources Online Spatial Data: Geologic maps (usgs.gov)
…And for the geeks. Click on the image for a full-screen view.
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