Geological Survey Professional Paper 729—B Volcanic Stratigraphy of the Quaternary Rhyolite Plateau in Yellowstone National Park

G. M. Richmond, K. L. Pierce, and H. A. Waldrop are studying upper Pleistocene surficial deposits which overlie the plateau volcanic rocks as well as sediments interlayered with the volcanics. Their studies will be reported in forthcoming publications. For this reason, the sedimentary stratigraphy of the rhyolite plateau is not treated at length in the present report; however, deposits intercalated with the volcanic units are mentioned briefly in this section.

Gravels and associated minor sands and silts in the Junction Butte Basalt, the sediments and basalts of The Narrows, and the Osprey Basalt have already been noted. All of these sediments originally were included in the Tower Creek Conglomerate as mapped by Hague, Iddings, and Weed in the Yellowstone monograph atlas (Hague, 1904) and by Brown (1961, p. 1182-151;1183). Deposition of these sediments was separated by emplacement of ash-flow sheets of the Yellowstone Group and by episodes of valley cutting, and the gravels contain different assemblages of coarse clasts. Pierce, Christiansen, and Richmond (1970), therefore, restricted the term "Tower Creek" to the canyon-filling sediments which contain Yellowstone Group clasts and used the name as a member of the Osprey. Because subsequent separation of the type Tower Creek Gravel Member from the Osprey introduces further confusion into the term, we no longer use it but regard the gravels informally as associates of the various basalts. Minor gravels also are interlayered with the Undine Falls Basalt.

On the south side of Mount Everts an alluvial channel gravel about 2 m thick, composed almost entirely of debris of the Absaroka Volcanic Super-group, underlies the Huckleberry Ridge Tuff and overlies Cretaceous sedimentary rocks. A wedge of loess lies between the Huckleberry Ridge and Mesa Falls Tuffs in the type section of the Mesa Falls.

Howard (1937) noted and described sediments in and near the upper Grand Canyon of the Yellowstone. In the area of the Grand Canyon, the Canyon flow is overlain by highly altered well-bedded sandstones and siltstones along the canyon rims; perhaps the best exposure of these sedimentary rocks is in cliffs about one-half kilometer northwest of Inspiration Point. Just above Lower Falls, sediments including a possible till of pre-Wisconsin age occupy a paleovalley cut in the Canyon flow and in the sediments of Inspiration Point and are overlain in turn near Upper Falls by clays, silts, sands, and partly cemented pumiceous sandstone (Richmond, 1970a; 1970b, p. 10). Sanidine from pumice in the basal beds of the sediments of Upper Falls has a K-Ar age of about 290,000 years; the sediments are overlain by a flow of the Central Plateau Member, sanidine from which has been dated as about 110,000 years (J. D. Obradovich, written commun., 1970). Two sediment-filled segments of a high-level channel cut off from the subsequently deepened main channel are stranded on the north wall of the Grand Canyon near the trail to the Red Rock overlook and about two-thirds kilometer west of Inspiration Point. These sediments of Red Rock are similar to those of Upper Falls but may be older. Sands, gravels, and tuffs in the Grand Canyon at Sevenmile Hole, about 8 km downstream from Lower Falls, may be partly equivalent in age to those in the area of Upper and Lower Falls and Red Rock (G. M. Richmond, oral commun., 1971).

At a gravel pit two-thirds kilometer northwest of Potts Hot Springs a flow of the Central Plateau Member is underlain by gravels and sands containing abundant clasts from the Central Plateau and Shoshone Lake Tuff Members. The sediments also appear to overlie the Shoshone Lake on a steep paleoslope.

A thin till just north of Bechler Meadows in southwestern Yellowstone National Park, about 2 km east of Boundary Creek, lies between the Falls River Basalt and a rhyolite flow of the Central Plateau Member.