Working On High Elevation Tertiary Strata, Southwestern Montana

Lion Mountain, south-central Gravelly Range in southwestern Montana, has about 300 m of Tertiary strata capped by basalt that is about 31 million years in age.

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.

Channel complex near top of Lion Mountain comprised of Paleozoic rock clasts.
Basal surge deposit about 25 m from top of Lion Mountain. Embedded basalt clasts, sand waves, and plane parallel beds characterize this deposit.
Basalt bombs in channel near Lion Mountain crest have paleomag drill holes – a clear sign that someone else has made this climb!
A ladder stashed in the uppermost tree-area on the mountain which is left over from past paleontology expeditions.
The orange baked zone that underlies basalt is evident in this photo. Also note the channel lenses that outcrop randomly across the Tertiary stratal expanse.
About 5 km northwest of Lion Mountain sits the basalt plug of Black Butte. Previous reported isotopic ages range from 23-25 million years, but our preliminary data show an age of about 30 million years for this volcanic feature.
The most pleasant part of the hike in the Lion Mountain area is in the glaciated meadow that lies at the base of the mountain. We’re a little late for the wild flower bloom, but it still is a gorgeous area!