Yellowstone Ash in Montana

Montana Geology with the Rock Doctor
Montana Geology with the Rock Doctor
3,416
0
Published 2024-06-13
This video covers Miocene (17 to 4.5 Ma) stream deposits of the ancestral Missouri River in southwest Montana. The deposits, named the Sixmile Creek Formation, help us to understand the importance of the track of the Yellowstone hot spot on the evolution of this drainage. Starting 17 million years ago with the outbreak of the hot spot near the collective borders of Nevada, Idaho and Oregon, thermal bulging of the crust generated the ancestral Missouri River, flowing to the northeast down extensional valleys or grabens formed by the bulging and stretching of the brittle crust. The drainage collected stream-deposited ash from caldera-forming eruptions of the hot spot, as well as distinctive far-traveled chert and quartzite gravel and locally-derived alluvial fan deposits. The ash can be found as far north as Hudson Bay!

Our research shows that the ash was likely deposited during caldera-forming eruptions that breached the walls of existing calderas, releasing ashy lahars or sediment-laden outburst floods down the Missouri River. As the plate moved over the stationary hot spot to the southwest, it arrived at its present location a few million years ago, resulting in a collapse of the thermally-bulged crust left behind, forming northwest-trending basins and ranges. The development of this topography, the drainage system we see today, caused the termination of the ancestral northeast-flowing pathways of the Missouri River, raising the old deposits into many of the northwest-trending mountain ranges in southwest Montana.

This video is focussed on spectacular exposures in the Sweetwater Range near the East Fork of Blacktail Deer Creek. I will not provide specific location data, since this area is highly sensitive, not only due to the fragility of the rocks, but the many bird species who nest here during the spring and early summer. An adventurous person has enough information to be able to visit the deposits, and so I ask that you tread lightly if you do so. There is a PowerPoint presentation on this YouTube site that goes into the details of this story, and I can share a PDF copy of a recent research article (Thomas and Sears, 2020) upon request. Thank you...Rob
All Comments (14)
  • @RockOcean
    Thanks Doctor Rock! Thoroughly enjoying your videos. Your camera work, audio, and commentary is great keep them coming. Not to mention now being able to better understand the natural environment that surrounds me. Thank you!
  • @nheffel2
    Great video. This helps answer a lot of questions, I had about the Ancestral Missouri River. Next time I’m down there, I will check out those features!
  • @terryolson196
    Loved this video! Never made it over to the hoodoos, might have to add it to my bucket list!
  • @yukigatlin9358
    😮✨Wow, spectacular geology hiding in the sedimentary layers of rock, Rock Doctor!! I have a new appreciation to the rock, thank you, Rob!!💛✨
  • @charleymitchell5461
    I always wonder what the drainage east of the Idaho batholith looked like before the hotspot blasted northeast. Thanks, I can kind of see it now.
  • @BeautifuLakesStreamsBiologists
    Thanks for this. We are working on the south side of the Clark Fork a half hour west of Missoula. The soil survey shows ashy soils. I was very happy to see this because we use ash to create unique aesthetics in our lakes. I had assumed it blew in off the Cascades, but I've not seen it yet. Looking forward to more ashy soils across Montana in the futre!
  • @conniewojahn6445
    Good thing you said that was a penny. It was the first thing I noticed when you showed the formation, and I thought it was a penny. Everything after the first notice up to the penny revelation, I wasn't paying attention to and had to replay. Attention span of a gnat! Nice close ups. Yes, I also wondered about the white, tabular, blocks. I presumed it was a clunk of mud consistency ash smeared somehow. The pulses explanation makes sense to me. Waiting for the explanation of the vertical lines/breaks, and the brown tinting that looks like a line of spray. Maybe I missed that when my bran muffin distracted me? I'll have to replay that, too. Always amazes me how far water can carry rock before depositing it, and the size/weight of the carried rock. People think of the little bit of area where they are, or what land they own, and don't think about the overall earth forces and distance spread across the earth. Geology takes in large areas as well as large blocks of time. Ah, there it is, contraction joints.