JACKSON, Wyo. — Yellowstone is one of the most iconic parts of Wyoming, a state whose identity is tied to the Rocky Mountain West. However, the geological history of the local landscape includes a time when Wyoming was at the equator and the Yellowstone hotspot was as far away as Oregon.
According to John Hebberger Jr., retired geologist and former vice president of Geologists of Jackson Hole, half a billion years ago the North American continent was at the equator. Wyoming was actually positioned very near the hemisphere divider, prior to the continent drifting north and rotating into the position it’s in today. From a little over half a billion years ago to around 50 to 65 million years ago, Wyoming was at sea level; oceans swept back and forth and deposited most of the sedimentary rocks seen locally today including carbonates, sandstones, siltstones, mudstones and shales.
“A very large percentage of the rocks that you see in the mountain ranges here and across the state were formed in oceans,” Hebberger tells Buckrail. “The oldest ones were formed during the Cambrian time, about 540 million years ago, and onward all the way up through the Cretaceous about 66 million years ago.”
Starting around 65 or 70 million years ago and lasting until about 45 to 50 million years ago, the Laramide Orogeny mountain-building event happened across Wyoming, progressing from the western to the eastern side of the state with rocks from 2.6 billion years ago through the Cretaceous Period. The event formed almost all of the mountain ranges across Wyoming, except for the Tetons, Absoraka and Wyoming Ranges.
Hebberger highlights how the Gros Ventre Mountains in particular make that geological history visible to locals and visitors today. The core of crystalline rocks have carbonate rocks, like limestone, deposited on top from hundreds of millions of years of tropical seas. Cretaceous marine rocks followed, including mostly marine mudstones, shales and sandstone. The younger rocks form the flanks on those mountains, as they were eroded off of the mountain crests.
Then, 16 to 17 million years ago, the Yellowstone hotspot began in what’s now Oregon, near the Nevada border. According to Hebberger, the hotspot moved east to where Yellowstone is today, a number of ancestral Yellowstones existing along the way.
“The Snake River Plain is the track of the Yellowstone hotspot,” Hebberger tells Buckrail. “There are ancestral Yellowstones, and there’s really good evidence for them that stretch all the way back to that Oregon-Nevada location.”
Two pyroclastic flow events from that hotspot are visible in Signal Mountain today. Hebberger explains that a pyroclastic flow event is when flows of very hot gas and ash from a volcanic eruption deposit a horizontal layer that welds together as a result of the intense heat, known as a welded tuff. The older event happened four and a half million years ago when an ancestral Yellowstone west of Wyoming created the Kilgore Tuff. The second event happened two million years ago in present-day Yellowstone, and created the Huckleberry Ridge Tuff.
Hebberger says the older tuff is dipping, or sloping, 22 degrees, and the younger one is dipping 11 degrees. The dips are indicative of beginning motion on the Teton Fault, which developed the down-dropping, or lowering, of the floor that became Jackson Hole.
But that down-dropping didn’t happen without further change to the local geology. The faulting that created what is today Jackson Hole caused the western end of the Gros Ventre Mountains to be lost, truncated after formerly extending into what is today eastern Idaho.
Stay tuned for an in-depth look into the specific geological history of the Gros Ventres, and the impact it has on the valley today.