My field work in the Gravelly Range of southwest Montana has been on-going for a few years. Each field season there usually are field crews from the Alf Museum/Webb Schools headed up by Don Lofgren, along with many other earth scientists from a variety of places working together in the Gravelly Range. We base our field camp at the US Forest Service’s Black Butte Cabin for part of July and August.
Initially we focused our work on Eocene-Oligocene vertebrates and their depositional environments. However, as our work progressed, we came across extensive travertine deposits. During the recent Geological Society of America’s Annual meeting in Denver, CO, we presented our work to date on the Gravelly Range thermal springs. Below are a few segments of our presentation.
Late Eocene-early Oligocene travertine deposits occur along the northern crest of the Gravelly Range, in the Greater Yellowstone region of southwestern Montana, at circa 2900 m in elevation.
The travertine deposits exist in three main areas that trend in a northwesterly direction for approximately 2.5 km and are up to 52.5 m in stratigraphic thickness. Extensional faulting and reactivated thrust faults control the spatial distribution of the travertine deposits. Late Eocene-early Oligocene strata age defined by vertebrate assemblages and volcanic units are lateral stratigraphic equivalents to the travertine deposits. Five distinct facies, along with their main identifying features, are delineated within the travertine deposits. Vent facies include mounds comprised of inclined, crenulated concentric laminae and botroidal structures; apron/channel facies contain streamer fabric and packed, fragmental textures; pool facies include bubble mats, wavy lamination with conical tufts, and foam texture; proximal slope facies have well-developed inclined terracetttes and microterracettes; and distal slope facies contain microterracettes, spherules, and encrusted plants. With the exception of the vent facies, plant fragments and leaf imprint assemblages of Eocene-Oligocene age are found throughout the travertines but are best preserved in the distal slope facies. A preliminary list of floral taxa include Metasequoia, Sequoia, Gingko, Quercus?, Fagopsis, Tilia?, Amelanchier/Rosa, Plafkeria?, and Cornus. The travertine deposits were previously mapped as Upper Cretaceous-Paleocene Beaverhead Formation but are herein reassigned to the southwestern Montana informal Cenozoic Sequence 2. These deposits are heretofore undocumented but now are recognized as the most complete Paleogene travertine depositional system in the Greater Yellowstone region.
Three main areas of the thermal springs (hover cursor over photos to see captions):
Photos of thermal springs features (hover cursor over photos to see captions):