My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 5th National Park along the way.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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My entrance photo shot. This one taken from the northern, Montana entrance, to the park since that is the only part of the park that we visited on this trip. This area is known as the North District.
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| Loading ramp at the Ok-A-Beh Marina |
Due to our limited time, and the large distance between districts, we only visited a small piece of this park. Here in the North District there was this boat ramp that went down into Bighorn Canyon and we could see some of the oldest rocks within the canyon. While there are older rocks in areas of the canyon outside of easy driving reach, these are the oldest accessible by car. At the boat launch here we have two rocks easily discernable. Along the water's edge is the Madison Limestone (the whiter layer) and just above that is the Amsden Formation. Topping the hills in this region is the Tensleep Sandstone.
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| A nice stratigraphic column of the park's rock formations from the NPS. |
As you can see by the stratigraphic column above, most of the rocks at this end of the park are towards the bottom of the rock record. The Madison Limestone (also known as the Madison Group) is an Early Mississippian Age limestone (~350 million years ago). At this time there was a shallow sea across the region where sea life built up creating the limestone. The Madison Limestone contains abundant fossils from this ancient sea as well as evidence of a ancient karst topography like sinkholes and caves (think modern day Kentucky).
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| A cross section of the park's formations. North is on the left with the Montana North District along the left 1/4 of the page. Image courtesy of the NPS. |
Above the Madison Limestone is the Amsden Formation. The Amsden Formation disconformably lies on top of the Madison Limestone and is of the earliest Pennsylvanian in age (~320 million years old). By lying disconformably, that means that there was a period of erosion between when the Madison Limestone was deposited and the Amsden Formation was deposited. This erosion produced the ancient karst topography that the Madison Limestone is known for.
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| View to the west (left) of the previous image at the Ok-A-Beh Marina. |
The Amsden Formation can be broken down into a few different beds, not all of which are represented in this area. In this region, the lowest (and oldest) is the Darwin Sandstone Member. This sandstone is a red and brown quartz arenite sandstone and is thought to have been deposited as a beach/shoreline deposit as the water levels were deepening in the area. As the water continued to deep, above the Darwin Sandstone, the Horseshoe Shale Member was deposited. The Horseshoe Shale is red to grayish red or purple siltstone, shale, and mudstone. These rocks were deposited in the deeper waters of a sea as it transgressed across Wyoming. And lastly, above the Horseshoe Shale is the Ranchester Limestone Member. The Ranchester is made up of yellowish-grey cherty dolomite and limestones, interbedded with sandstone and shale.
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| Looking east, near the easternmost extent of the park, midway along OK-A-Beh Road. |
From this view we can see several of the next overriding layers. The road is currently now sitting on the Tensleep Sandstone with the next overriding layer the Triassic Age (~250 to 200 million years ago) Chugwater and Goose Egg Formations at the base of the hill in the distance. Then we have a swift succession of thinly bedded Jurassic Age (~200 to 145 million years ago) formations including, from bottom to top, the Piper Formation, the Rierdon Formation, the Swift Formation, and the Morrison Formation. Capping off the hill is the Cretaceous Age (~145 to 66 million years ago) Kootenai Formation and Thermopolis Shale.
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| Geological Map of the Ok-A-Beh Marina area of Bighorn Canyon NRA. Map courtesy of the NPS. |
These rocks are all thinly bedded sandstones, siltstones, shales, and limestones, that alternate through time. These deposits represent the inland sea as it covered the area and then left the area multiple times with lake and river deposits mixed throughout. The Morrison Formation itself is a world famous dinosaur fossil hotbed that represents terrestrial river deposits during the Jurassic Period.
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| Extent of the Laramide Orogeny. Image courtesy of geology.wisc.edu. |
Following the deposition of these rocks, they were then lifted into the nearby Bighorn Mountains by what is called the Laramide Orogeny that began roughly 70 million years ago through 40 million years ago and had impacted the landscape across the North American west from mid-Montana down through New Mexico. The Laramide Orogeny, or mountain building episode, was caused by the former Farallon Plate off the western coast of North America pushing eastward, causing the compression of the North American Plate and mountains to be forced upwards. The Farallon Plate would late completely subduct beneath North America and would be impactful in forming many National Parks across the Colorado Plateau.
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| A little further downstream from the Ok-A-Beh Marina. |
As the Bighorn River was eroding the region, the landscape continued to be lifted upwards. The river now contains what are known as "entrenched meanders". These are when a river was in a formerly fairly level plain and allowed to meander back and forth depositing sediment along a neighboring floodplain. However, the landscape is then suddenly lifted upwards, changing a river that was depositing sediment in a floodplain to an eroding river. The river then cuts downwards into the rocks that are now being forced upwards. This downward erosion is in the shape of the meandering river, causing the meanders to become locked in place, an entrenched meander. The meanders of the Bighorn River were locked in place during the Laramide Orogeny and have been eroding downward steadily ever since. A more famous example of an entrenched meander can be seen in the Grand Canyon or Dead Horse Point.
This last image is just upslope of the Ok-A-Beh Marina and to the left as you follow the Bighorn River. Again you can see here the Madison Limestone just above the river line with the Amsden Formation (the red rocks) on top and the Tensleep Sandstone capping off the hills.
References
Garber, K. L., et al. "Detrital zircon U-Pb geochronology and provenance of the basal Amsden Formation." Bighorn Mountains: Wyoming Geological Association 72nd Annual Field Conference Guidebook. 2018.
