Date of this Version
Seven sequences of Pleistocene strata, five of them predominantly lacustrine, are described from outcrops north of Yellowstone Lake. These are (1) Turbid Lake sequence, 30–50 feet of white pumiceous claystone and tuff with carbonaceous partings and a distinctive compositional pattern of excesses and deficiencies of many elements; (2) Yellowstone Falls sequence, 75 feet or more of varved white claystone and tuff containing pollen and diatoms, overlain by gray conglomerate and sandstone; (3) Hayden Valley sequence, 200 feet or more of gray and white silt and claystone containing sparse diatoms; (4) Alum Creek sequence, 30 feet or more of bedded sand and gravel with lesser amounts of clay; (5) Mudkettle sequence, 150 feet or more of light gray to white, soft clay and claystone with lesser amounts of sandstone and conglomerate, moderately lithified in part, and containing some pollen and diatoms; (6) Astringent Creek Sand (newly named), a gray, commonly unlithified sand as much as 300 feet thick and containing abundant volcanic debris; and (7) Pelican Valley sequence, 120 feet or more of light gray to white, soft clay, silt, sand, and some pumice pebble conglomerate and shard beds; finer grained beds contain diatoms, pollen, and carbonaceous debris that has a radiocarbon date of 7,550 ±350 years.
Other localized deposits with radiocarbon dates and abundant diatoms consist of white carbonaceous tuffaceous clay and sand with an age of 9,440±300 years, in Gibbon Canyon, and a gray and white carbonaceous clay, silt, and sand with an age of 3,750+300 years at Bannock Ford in Yellowstone Canyon.
Slight arching of the Upper Basin Member of the Plateau Rhyolite caused the Yellowstone River to develop an antecedent course across it. Uplift of the Pelican Valley area during the last 7,500 years averaged about one foot in 50 years.