AGE CONSTRAINTS ON CENOZOIC CONTINENTAL DEPOSITS, MADISON BLUFFS, SOUTHWESTERN MONTANA

The exposed portion of the Cenozoic section in the Madison Bluffs (Gallatin County) of southwestern Montana is about 700 feet in thickness, with strata outcropping for approximately 26 kilometers along the lower east side of the Madison River. These exposures provide the thickest and most continuous set of Cenozoic strata in the Gallatin-Three Forks valleys, and as such, represent an important locale for establishing areal Cenozoic age constraints. The Madison Bluff exposures potentially contain Chadronian to Clarendonian age deposits based upon a few fossil vertebrate collections, most of which have no specific locality data. The main fossil vertebrate assemblage that age constrains any part of the bluff section was collected by Earl Douglass, mainly from 1894-1896, and is now housed at the Carnegie Museum of Natural History. It has been assumed that these fossil vertebrates were collected near the top of the Madison Bluff exposures, but no exact locations were known. Our recent field work in this area, done in conjunction with correlating Douglass’s original field notes with various topographic features, provides relocations for Douglass’s fossil vertebrate collection localities. The main locality collected by Douglass is atop Big Round Top, a prominent hill located midway along the Madison Bluff exposures. The relocation of Douglass’s collection sites places an age constraint of Early Barstovian to potentially as young as Clarendonian for strata approximately 170 feet below the top of the Madison Bluff’s aggregate measured Cenozoic section. This age constraint for a part of the Madison Bluff’s Neogene strata provides a correlation point with the well-known Anceney fossil vertebrate locality also located in the Gallatin Valley, about 18 kilometers southeast of the Madison Bluffs, as well as other Neogene deposits in southwestern Montana.