Heart Mountain, Wyoming Revisited
Heart Mountain, a 8,123 feet high peak, is composed of dolomite,limestone and volcanic rocks formed several hundred million years ago, and is lying atop, and surrounded by, younger rocks formed around 55 million years ago. Older rocks atop younger rocks rarely happen, but do occur in geologic features such as igneous intrusions into older rock, recumbent folds, thrust faults, detachment faults, and -spoiler alert- landslides.
But how exactly this mass of older rocks, part of a much larger mass of rocks, managed to overcome gravity and slide 28 miles in under 30 minutes became the controversy of its day. Turns out, Wyoming geologist Myron Cook has the answer due to tons of work by hundreds of geologists, and it is a much larger story than just Heart Mountain. The link below is to his fascinating, but very long video on the Heart Mountain Detachment fault and landslide. I encourage you to watch it, but have recapped it below.
The first image, a Google Earth map below, shows the area of Wyoming near Cody where the landslide occurred. The arrows point to two volcanic systems, even larger than Yellowstone, that lay northwest (map looking to the southeast towards Heart Mountain, small green dot) that existed around 50 million years ago. These stratovolcanoes were 20,000 feet high above the Bighorn basin, and the magma fed up dikes through the hundreds of millions year-old sedimentary rocks.
These volcanoes were explosive and the earthquakes they generated were enough to “rattle things around” and crack open a detachment fault along a bedding plane of the Bighorn Dolomite. Recent research suggests the detachment was caused by collapse of one of the volcanic flanks. Nevertheless, the red outline shows the area of the Heart Mountain detachment/ landslide zone along which about 20 miles by 30 miles of 800 + feet of sedimentary rock topped with (maybe) thousands of feet of volcanic deposits that slid catastrophically, very quickly, and all at once.
The second image shows the detachment fault, a hairline thin contact in the dolomite (carbonate rock) with a very shallow dip of only 2%, just feet above a shale horizon. Once movement was initiated, the mass began to slide. The heat from the friction extracted and superheated CO2 gases buoying the rock. The shallow slope became “frictionless” and in areas there is no deformation of the underlying base.
The slide caused the mass of rock to break into blocks, some have estimated over 100 blocks slid during the slide at a speed well over 100mph. The directions of the sliding bifurcated, (image 3) and some blocks slid towards where Heart Mountain stands now, and some slid more southerly where other blocks now lie.
The last image (4) shows something startling - if you drilled through Heart Mountain down into Bighorn Basin, you would reach Bighorn dolomite again 15,000 feet down, and mirroring the stratigraphy of Heart mountain. This is because between 75 and 50 million years ago, a period of mountain-building called the Laramide Orogeny caused uplift of the area the detachment formed in, and subsidence of the Bighorn basin. The Bighorn basin filled with thousands of feet of newer sedimentary rock including the 55 millon year-old Willwood formation on which Heart Mountain lies.
Learn how Supervolcanoes caused the World’s Largest Landslide in Wyoming by Myron Cook: https://youtu.be/CYS3r3tk2GI
Here is some more technical information for those so inclined: http://faculty.mnsu.edu/stevenlosh/research/block-sliding-heart-mountain-detachment-wyoming/
And more recent info here: https://www.researchgate.net/publication/254617445_Recent_Contributions_to_the_Understanding_of_the_Heart_Mountain_Detachment
Pinging @BoxcarMurphy again. Sorry Boxcar!
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