Tertiary Geology and Paleontology in the Gravelly Range, Southwestern Montana

Lion Mountain in the Gravelly Range of southwestern Montana. This area is federal land managed by the U.S. Forest Service.

A part of my recent geological field work includes working on high elevation Tertiary strata in the Gravelly Range, southwestern Montana. The Gravelly Range is located in southwest Montana, about 10 miles southwest of Ennis, Montana. For some background on this area and what my field work is about, see an older blog that I posted at Geopostings.

So – now that one field season is done and field data compiled, both my co-worker, Don Lofgren and myself have interpreted some of our data. We recently outlined our work at the Geological Society of America’s (GSA) Rocky Mountain section meeting in Calgary. Alberta. The abstract from our session is given below as well as the poster itself in both a jpeg format and as a link to our  GSA presentation.

“Tertiary strata exposed in four high elevation areas in the south-central
Gravelly Range yield significant assemblages of Late Eocene to Oligocene
mammals. The thickest stratigraphic sections of Tertiary strata are in the
Lion Mountain-Black Butte area. The Lion Mountain section age is based
primarily on American Museum of Natural History collections; the lower
part of this section is Duchesnean-Chadronian (39-33 Ma) and the
uppermost beds are Whitneyan (32-31 Ma). Age of the basal part of the
Black Butte section is Duchesnean-Chadronian based on Harvard Museum of Comparative Zoology collections. Recent collections that include Miohippus indicate a probable Orellan age for uppermost exposures. The Tepee Mountain section is notable for abundant brontothere remains and is probably Duchesnean-Chadronian (approx. 39-33 Ma). The Rapamys site is the oldest vertebrate locality and is late Uintan to early Duchesnean (42-38 Ma) based on recently recovered specimens of RapamysProtoreodon, and Lycophocyon.

The Tertiary strata in this part of the Gravelly Range include fluvial, aeolian, and tufa deposits that are most likely mainly associated with localized Oligocene volcanism. The Lion Mountain section is about 270 meters in thickness; the lower half of the section is largely aeolian, with fluvial units comprising much of the upper section. Based upon age data, the 140 meter Black Butte section correlates to the lower 50-70 meters of the Lion Mountain section. The basal 20 meters of the Black Butte section contain some fluvial features, but much of the remaining section is largely aeolian in origin. Paleosols and extensive burrowing also occur within the Black Butte section. Stratigraphic section thickness decreases rapidly away from the Black Butte-Lion Mountain area, with section thicknesses of about 20 meters for the largely aeolian Rapamys and Tepee Mountain sections. Tufa deposits are located along the west-central edge of the Gravelly Range where they are associated with previously mapped thrust faults. Leaf imprint assemblages of Eocene-Miocene age are contained within these tufas. Strata previously mapped as Upper Cretaceous-Paleocene Beaverhead Formation are now variously reassigned to the lower Cretaceous Kootenai Formation, southwestern Montana Cenozoic Sequence 2, and diverse Quaternary units.” From: Abstract from Geological Society of America Abstracts with Programs. Vol. 49, No. 5 doi: 10.1130/abs/2017RM-293156.

The poster presented at the 2017 Rocky Mountain GSA is available below as a jpeg and at GSA as a pdf.

Cenozoic Sequence Stratigraphy of Southwestern Montana

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:

The Field Season Is Going Strong in Southwestern Montana

My field season is in full swing. I recently spent time with students from the Webb Schools in Claremont, CA, during their annual sojourn to southwestern Montana. We prospected a few Tertiary localities, with the students making some good fossil mammal and fossil invertebrate finds. We were also extremely lucky to have a southwest Montana landowner give us a tour of a buffalo jump that is on his land. The following photos are from our various fossil site and buffalo jump field adventures.

woodin-snails
Tertiary fossil snails (about 25 My in age) at one locality captured the interest of students. Once one snail was found, everyone was intent on finding more.
Bob Haseman talks about a buffalo jump in the Toston Valley. He is standing by one of the many tepee rings associated with the jump site.
Bob Haseman talks about a buffalo jump in the Toston Valley of southwestern Montana. He is standing by one of the many tepee rings associated with the jump site. The small boulders on the surface between Bob and the students are part of a tepee ring.
Webb School students hiking up to the "Looking-Out" site associated with the buffalo jump. A eagle catchment area is immediately below the highest point of the "Looking-Out" site.
Webb School students hiked up to the “Looking-Out” site associated with the buffalo jump. A eagle catchment area is immediately below the highest point of the “Looking-Out” site.
eagle-catchment
The eagle catchment area is a shallow depression where a person would hide beneath brush awaiting the approach of an eagle. A nearby animal carcass would aid the quest to capture a eagle which was then used for its feathers.
Chadronian (about 36 Ma) age rocks yielded a few brontothere teeth and bone fragments.
Chadronian (about 36 My in age) rocks near Three Forks, Montana yielded a few brontothere teeth and bone fragments for the curious students.
Chadronian strata in this area contain brown to reddish, popcorn textured floodplain deposits and whitish-colored fine-sand channel deposits.
Chadronian strata in this area consist of brown to reddish popcorn-textured floodplain deposits that contain paleosols and whitish-colored fine-sand channel deposits.

 

 

A High-Elevation Eocene Fossil Vertebrate Site in the Elkhorn Mountains, Southwestern Montana

dogtown1Af
The Dog Town Mine vertebrate fossil locality is an isolated occurrence of Eocene strata found on the divide between the Toston-Townsend Valley (on the east side of the photo)  and the North Boulder Valley (on the western edge of the photo), southwestern Montana.

The Dog Town Mine Tertiary fossil vertebrate locality is nestled on private property within the southern extent of the Elkhorn Mountains, southwestern Montana. The locality is about 20 miles southwest of Townsend, Montana, where Mesozoic and Paleozoic carbonate, quartzite, and red-colored mudstone, siltstone, and sandstone rocks underlie Eocene (Chadronian) strata. These unconformable Eocene strata contain the Dog Town Mine vertebrate fossil locality.

Earl Douglass (yes, that Earl Douglass of the Dinosaur National Monument fame) first collected at the site on Friday, June 27, 1902 (based on transcriptions from Earl Douglass’ journals done by Alan Tabrum and volunteers from the Carnegie Museum of Natural History). According to his journal, Douglass met a man from Toston, Montana, on horseback and this person told him about the Dog Town Mine, which was located on the divide between the Toston/Townsend and North Boulder Valleys. Douglass was headed to the North Boulder Valley anyways, so he rode to the mine where he found invertebrate fossils (brachiopods and bryozoa) in carbonate rock which was in contact with the ore deposit. A Mr. Allen, who he dined with that evening, told him that more fossils could be found a little ways west of the mine. After dinner Douglass rode a short way west of the mine and found banks along a ravine that looked like Tertiary White River beds. Here he found  “Oreodont, Ischyromys, Palaeolagus, Titanotherium, and turtle remains” (June 28, 1902, Douglass Journal entry). This area is the present Dog Town Mine vertebrate fossil locality.

The Dog Town Mine site encompasses all of the light-colored exposures on the right side of the county road.
The Dog Town Mine locality encompasses all of the light-colored exposures on the right side of the county road seen in this photograph.

 

Tertiary strata at the Dog Town Mine are fine-grained, predominantly consisting of siltstone with minor fine-grained sandstone units. The deposits are probably of aeolian origin, originating from areal sediments rich in volcanic ash. These deposits are probably similar lithologically and in mode of origin to those Tertiary White River units found at high elevations within the Laramie Range and Medicine Bow Mountains (Evanoff, E., 1990, Early Oligocene paleovalleys in southern and central Wyoming: Evidence of high local relief on the late Eocene unconformity: Geology, v. 18, p. 443–446; Lloyd and Eberle, 2012, A late Eocene (Chadronian) mammalian fauna from the White River Formation in Kings Canyon, northern Colorado: Rocky Mountain Geology, v. 47, no. 2, p. 113–132).

Vertebrate fossils have been collected at the Dog Town Mine site for various museums since Douglass’ initial collection. The Carnegie Museum of Natural History in Pittsburgh, PA houses a collection from the site as well as the Museum of the Rockies in Bozeman, MT.

Earl Douglass and the Tertiary Geology of Southwest Montana’s Madison Bluffs

Most vertebrate paleontologists probably think of the spectacular dinosaur finds near Jensen, Utah, when the name Earl Douglass is mentioned. Douglass’s discovery of a partial Apatosaurus near Jensen in 1909  did spark the beginning of his long career with finding more dinosaur material in what we now know as Dinosaur National Monument. But Douglass began his quest for fossil vertebrates while he was in southwestern Montana – several years before he was summoned by the Carnegie Museum of Natural History’s director William Jacob Holland to find dinosaurs.

From the spring of 1894 to 1896, Douglass taught at a one-room school in the lower Madison Valley of southwestern Montana. The school house was located in the lower Madison Valley, directly west of the area known as the Madison Bluffs. These bluffs contain strata that range in age from probably as old as Eocene through the late Miocene. The strata are continental units that include alluvial fan to fluvial trunk stream deposits.

The school house near the Madison Bluffs, southwestern Montana, that Earl Douglass taught at from 1894-1896.
The school house near the Madison Bluffs, southwestern Montana, that Earl Douglass taught at from 1894-1896.
The Madison Bluffs consist of Tertiary fluvail/alluvial fan strata of probably Eocene to late Miocene age.
The Madison Bluffs consist of Tertiary fluvial/alluvial fan strata of probably Eocene to late Miocene age. The Madison Buffalo Jump State Park is located at the northwest edge of this photo.

During his tenure at the lower Madison Valley school, Douglass spent much of his spare time exploring the Madison Bluffs. At the beginning of his teaching contract in 1894, he had very little knowledge of vertebrate paleontology and of the area geology. He initially considered the Madison Bluff beds as Cretaceous in age. But when he found a “tooth very much like a Protohippus” (Earl Douglass journal entry on May 12, 1894), Douglass knew that the beds were younger in age. As time passed, he began to find a significant quantity of fossil vertebrate mammal material within the bluff’s deposits. Consequently, he immersed himself into reading about comparative anatomy so he could readily identify the fossil material. Douglass eventually used his collected fossil material for his 1899 Master’s thesis at the University of Montana – ostensibly the first Master’s degree awarded by the University.

horse jaw from douglass madbluff

Douglass kept journals of his time in the lower Madison Valley, and often detailed both the area geology as well as his fossil finds. Alan Tabrum and volunteers from the Carnegie Museum of Natural History have transcribed many of his journal entries from southwestern Montana. I’ve included two portions of journal entries to illustrate his finding of a horse jaw from the bluffs (above diagram) and one of Douglass’s drawings of “Big Round Top” (an area in the bluffs near the one-room school house) as compared to that same area today in a photo that I took about a week ago.

earldouglass_bigrt

It’s not difficult to understand how Earl Douglass became enthralled with the geology and paleontology of the Madison Bluffs. In addition to the fossil vertebrates, the bluffs contain many other fascinating geological features. Towards the central part of the bluffs (immediately south of the Madison Buffalo Jump State Park), calcic paleosol stacks mark the boundary between most likely Eocene and Miocene strata. The calcic paleosol stacks contain at least two generations of soil profiles (typically minus the A and upper part of the B horizons). Rootlets and burrows are commonly associated with these paleosols.

Volcanic tuffs also occur within the bluff’s strata, which is really handy for those of us who like isotopic age control for southwestern Montana Tertiary deposits. The tuffs could potentially help age constrain the paleosol stacks and sedimentation within the so far non-fossil bearing part of the bluffs. And with the help of the New Mexico Geochronology Lab, a group of us are working on just that aspect of Madison Bluff geology.

Calcic paleosol stacks in the central part of the Madison Bluffs, southwest Montana.
Calcic paleosol stacks in the central part of the Madison Bluffs, southwest Montana.
Roots within the calcic paleosols found at the Madison Bluffs.
Roots within the calcic paleosols found at the Madison Bluffs.
Burrows at the base of a calcic paleosol.
Burrows and roots at the base of a calcic paleosol.
Gray tuff found below calcic paleosol stacks.
Gray tuff found below the calcic paleosol stacks.

The Gravelly Range, Southwestern Montana: High Elevation Tertiary Rocks

The Gravelly Range is located in southwest Montana, about 10 miles southwest of Ennis, Montana. Much of the range is covered by the Beaverhead-Deerlodge National Forest. The Axolotl Lakes Wilderness Study Area, managed by the Bureau of Land Management, is in the northern part of the Gravelly Range.

Gravelly Range - looking east over Paleozoic rocks to the Madison Range in the far distance.
Gravelly Range – looking east over Paleozoic rocks to the Madison Range in the far distance.

Our field group was interested in looking at Tertiary rocks, so we headed for the Black Butte – Lion Mountain area, the more south-central part of the range. A cold front had just swept through western Montana a few days prior to my field trip. That storm left some snow up on the range crest – yep, that’s right, snow in July. But it did melt off fast and it left vegetation along the Gravelly Range road (the main road that stretches along much of the top of the range’s extent) extremely lush. So it was a gorgeous drive from the Lyon Bridge crossing on the Madison River up to Lion Mountain and Black Butte. And as Black Butte is the highest peak in the Gravelly Range at 10,542 feet in elevation, it was not difficult to find our destination.

Black Butte, at 10,542 feet  in elevation, is the highest peak in the Gravelly Range.
Black Butte, at 10,542 feet in elevation, is the highest peak in the Gravelly Range. Eruptions at Black Butte have a radiometric age date by whole-rock K-Ar of 22.9 Ma.
East side of Lion Mountain as seen from Wolverine Basin. Alkaline basalt caps Lion Mountain, with a K-Ar age date of 30.8 Ma.
East side of Lion Mountain as seen from Wolverine Basin. Alkaline basalt caps Lion Mountain, and has a K-Ar age date of 30.8 Ma.

The Tertiary rocks of interest to us were primarily the Tertiary strata exposed on the west side of Lion Mountain. Fossil fauna from these strata have a North American Land Mammal Age of Whitneyan, and are approximately 29 to 32 million years in age. Carnivore, rodent, insectivore, and rabbit are some of the fauna of the fossil assemblage collected here by past workers.

The west side of Lion Mountain with Tertiary strata exposed under the 30.8 Ma basalt cap.
The west side of Lion Mountain with Tertiary strata exposed under the 30.8 Ma basalt cap.

It was a good workout to reach the top of Lion Mountain, but really was well worth the effort. The Tertiary strata had plenty of features to keep a sedimentologist like myself busy. And the views – just spectacular! To top off the trip – it was obvious that someone had been there before us because we found an aluminum ladder stashed is the trees near the top of the Tertiary exposures. None of us availed ourselves of its use, but maybe next time it will come in handy!

A ladder stashed in the bushes near the top of Lion Mountain. The Snowcrest Range is shown in the distance on the left hand side of the photo. Black Butte pops over the ridge in the photo's upper right.
A ladder is stashed in the trees near the top of Lion Mountain. The Snowcrest Range is shown in the distance on the left hand side of the photo. Black Butte pops over the ridge in the photo’s upper right.

 

 

 

Prospecting For Vertebrate Fossils On Hot Summer Days In Southwestern Montana

The end of July always brings The Webb School students who are interested in paleontology to southwestern Montana. That time is packed with prospecting a variety of Tertiary sites in the hopes of finding interesting vertebrate fossils. This year had its good finds along with persevering through some really hot days. Being on a surface of light-colored rocks under the intense sun while slowly looking for fossils such as rodent jaws, rabbit teeth, or even isolated horse teeth is a tough way to spend a summer day. Even prospecting for larger pieces of fossil vertebrates is no easy day, but the students hung in there. Here’s a few scenes from the prospecting adventures:

The late Eocene strata at Pipestone held the students attention for many hours
The late Eocene strata in the Pipestone area west of Whitehall held the students attention for many hours.

 

Late Tertiary outcrops north of Whitehall, Montana, yielded some interesting horse and camel remains.
Late Tertiary outcrops north of Whitehall, Montana, yielded some interesting horse and camel fossils.

 

A horse jaw and at least part of the skull was found in late Tertiary strata.
A horse jaw and at least part of the skull were found in late Tertiary strata located near Whitehall.

 

Still searching for that elusive fossil....
Still searching for that elusive fossil in the North Boulder Valley….

 

Late Tertiary canid dog skull fossil find from last season's Webb School  paleo students' efforts. Skull is now at the Museum of the Rockies.
Late Tertiary canid dog skull fossil find from last season’s Webb School paleo students’ efforts. The skull is now at the Museum of the Rockies in Bozeman, Montana.

 

 

 

 

 

 

 

For those truly interested in vertebrate paleontology, keep in mind that the renowned Raymond Alf Museum is also on the Webb Schools campus. The museum is definetly worth a visit.