Whenever I drive to Yellowstone National Park’s northern gate, I pass by the Devil’s Slide. It seems that the slide is my gate keeper to the park, and it is always fun to see it in all our different seasons. And once again, during a chance conversation in the park, I was asked about the geology of Devil’s Slide. Because of that conversation, I thought that I’d spend some time blogging about the slide’s geology.
Devil’s Slide is a part of Cinnabar Mountain, which contains steeply-dipping to overturned Paleozoic and Mesozoic strata. Cinnabar Mountain is fault-bounded on its north side by the Gardiner Fault, a north to northeast dipping reverse fault zone. At Cinnabar Mountain’s north end, the Gardiner Fault juxtaposes Archean crystalline rock (now partly masked by Tertiary intrusive rocks and Quaternary glacial sediments as shown on geologic map snapshot below) on the fault’s northern, up-thrown side against Paleozoic strata on its down-thrown, southern side. The Paleozoic-Mesozoic strata in Cinnabar Mountain are contorted because of drag associated with the Gardiner Fault.
According to Marius Campbell and others (1915, p. 92), “Cinnabar Mountain was named in the early days, when the bright-red streak that marks it from top to bottom was supposed to be due to the mineral cinnabar, a red ore of mercury.” (From: Guidebook of the Western United States: Part A – The Northern Pacific Route, With a Side Trip to Yellowstone Park, U.S. Geological Survey Bulletin 611). We now know that the bright red streak is not cinnabar (a brick-red form of mercury sulfide), but the area of red in Devil’s Slide is actually a set of Triassic age red beds that mark widespread continental deposition and limited marine incursions throughout the Rocky Mountain region. The red beds in this case get their color from the oxidation of iron-rich minerals contained within the rocks.
And now for the jumping fox and its association to my Devil’s Slide discussion – as I said previously in this blog, the conversation that I had with a fellow-park goer a few days ago brought about my blog on Devil’s Slide. My conversation about the slide happened while I was watching a fox hunt rodents in YNP’s Round Prairie, a gorgeous meadow near Pebble Creek Campground in the park’s northeastern area. The female fox hunted for hours that morning, and several photographers and myself were enthralled with her hunt. The light snowfall of the night before accentuated the bushy fall coat of the fox and gave the hunting scene great color contrast. Here are are few photos from the hunt:
Working on Tertiary strata in the Gravelly Range, southwestern Montana, is sometime daunting to do. The Lion Mountain Tertiary section shown in the photo to the right is one of those places that makes for a grueling day or several days of field work. The Tertiary section unconformably overlies various Paleozoic units, such as Mississippian Madison Group carbonates, Pennsylvanian-Permian quartzite, and Triassic carbonates and red mudstone. And the ascent from these pre-Tertiary rocks to the top of the Tertiary section is worth it – for both vertebrate paleontology and sedimentary features. Current work status in the project that I’m working on with the Raymond M. Alf Museum, Claremont, CA, is that the section contains vertebrates ranging in age from about 40 million years to about 31 million years in age. A tuff unit near the top of the section that we collected has an Ar/Ar age of 31.4+- 0.7 million years. The capping basalt (the dark zone on the top of Lion Mountain) has a reported K-Ar age of 30.8 +- 0.7 million years. Sedimentary features include massive aeolian units and some channeling near the top of the section. A basal surge deposit occurs about 25 m below the capping basalt, signalling the initial pulse of extensive basaltic volcanism in the Lion Mountain locale. Several photos of my most recent Lion Mountain climb illustrate the section’s features and are shown below.
Much of my research has been focused on Cenozoic sequence stratigraphy of continental basin-fill in southwestern Montana. This approach to the stratigraphy of continental deposits has facilitated correlation of stratigraphic units both within and among the various basins of this area. I recently gave a talk about my work in this area at Montana Tech of the University of Montana. Here’s the You Tube version of my talk: