Sheet Rainbow Tullock Formation
The Big Horn Mountains 60 miles to the west supplied the sediment of this exposed section of Tullock Formation (Fm). Tullock Fm. consists of alluvial fan and swamp deposits all the way back to the mountains. Sediments washing off the newly risen peaks were filling up the coterminously formed sedimentary basin (Powder River Basin). The huge coal mines we have here are mining the coal formed in those swamps at the base of the Proto-BigHorn Mountains. Those mountains were much higher when they were young plus the basin was deeper.
Huge blocks of the earths crust uplifted and correspondingly downwarped during a major tectonic compression episode called the Laramide Orogeny. Cloud peak is 13,175 feet and is visible in this image. The same compressional forces that uplifted the peaks, also downwarped the adjacent basin to the east. This Basin called the Powder River Basin. This basin the major source of coal in the US. The burning of this coal generates 30 percent of the electricity generated in the United States.
My ranch coincidentally sits directly on the western most edge of the Wyoming Black Hills. It is actually JUST east of the edge of the Powder River Basin. If I drive 2 miles west, I start to see alluvial fan sediment. These sediment fans stretch all the way from the Big Horns. Dissected into ridges by huge rivers washing off the peaks during glaciation. . These alluvial deposits are far reaching, called the “Tullock/Fort Union” formation. Major Mountain sized Anticlines and Synclines resulted from the continental wide compression.. Huge were the forces bending even the underlying crystalline Pre-Cambrian rocks. The rocks to clay washing off of those peaks filled the basin and washed just about to my front door.
Understanding the geology here takes many books to read, its a long list and growing 🤔😀📷
Location: near the Bliss DInosaur Ranch, WYoming/Montana borderlands
Title: Sheet Rainbow Tullock Formation