Pg. B5-B7. Mesa Falls Tuff of Yellowstone Group. Recognized in Island Park area and Centennial Mountains, Idaho, and southern Madison Range, Montana. Not exposed in Yellowstone National Park. Consists of basal air-fall pumice overlain by ash-flow tuff. Thickness 100+ m. Overlies Huckleberry Ridge Tuff; underlies Lava Creek Tuff. Age is Pleistocene; K-Ar age (sanidine) of 1.2 Ma (J.D. Obradovich, written commun., 1970).
Named from Upper Mesa Falls on Henrys Fork of Snake River, Island Park area [in W/2 SW/4 sec. 13, T. 10 N., R. 43 E., approx. Lat. 40 deg. 11 min. 24 sec. N., Long. 111 deg. 19 min. 48 sec. W., Snake River Butte 7.5-min quadrangle, Targhee National Forest, Fremont Co., eastern ID].
Type section: roadcut on U.S. Highway 20, about 6.8 km north of Ashton, [Lookout Butte 7.5-min quadrangle], Fremont Co., eastern ID.
Principal reference section: cliffs along gorge of Henrys Fork at Upper Mesa.

Source: Publication; US geologic names lexicon (USGS Bull. 1520, p. 201-202).

Recent petrographic and chemical studies of ash samples called Pearlette have shown that there are at least three different ash beds on the Great Plains and Rocky Mountain area of closely related petrographic and properties that are related to the Yellowstone Tuff. Pearlette type O identified at Onion Creek, SE1/4 NE1/4 sec 26, T24S, R24E, Grand Co, UT, Paradox basin, has 0.7 +/-0.2 m.y. age and at three localities in Meade Co, KS, Anadarko basin has 0.6 +/-0.07 to 0.9 +/-0.25 m.y. age. Type O correlates with Lava Creek Tuff of Yellowstone area WY. Pearlette type S is correlated with the Mesa Falls Tuff of Yellowstone area of 1.2 m.y. age. Zircons from type S not dated because of small size and paucity. Pearlette type B identified at Cudahy ash Mine, Meade Co, KS in the Anadarko basin has an age of 2.0 m.y. and is correlated with Huckleberry Ridge Tuff of Yellowstone National Park, WY in the Yellowstone province.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Does not crop out in WY, but Pearlette type S ash, its downwind equivalent has a K-Ar age on sanidine of 1.2 m.y. and is present in the Laramie and Hanna basins of the Greater Green River basin.

Source: GNU records (USGS DDS-6; Denver GNULEX).

Mesa Falls ash bed. The informal designation for ash that is the downwind equivalent of the Mesa Falls Tuff of Yellowstone Group of the Yellowstone National Park area, Wyoming and Idaho. Previously called Pearlette type S ash. Age is Pleistocene, based on K-Ar age (sanidine) of 1.27 Ma for Mesa Falls Tuff (J.D. Obradovich, written commun., 1979; calculated using current decay constants of Steiger and Jager, 1977); Plio-Pliocene boundary placed at 1.8 Ma.
Mesa Falls ash bed identified w/certainty in: Butler, Cedar, and Harlan Cos., NE; Albany Co., WY.

Source: Publication.