Quaternary Stratigraphy and Chronology In the American Falls Area , Southeastern Idaho

New information about the stratigraphy, paleoecology, and geochronology of the American Falls area in southeastern Idaho adds to and modifies previously published interpretations of the late Quaternary geologic history. Based on evidence from the Bonneville basin, the Bonneville Flood occurred about 14,000-15,000 years ago, rather than 30,000 years ago as suggested by previous researchers in the American Falls basin. This revised age for the flood is in accord with geologic evidence from the American Falls basin as estimated from stratigraphic relationships with loess deposits and alluvial gravels of late Pleistocene age. Radiocarbon dates from the flood deposits are all too old, probably because the dated materials were reworked from older deposits by the flood. The basalt of Portneuf Valley, which was dated previously by radiocarbon as 30,000 to 35,000 years old and therefore was presumed to be only slightly older than the flood, has a potassium-argon age of 0.583 * 0.104 million years, which is also supported by the stratigraphy of loess and buried soils that overlie portions of the flow. The flood exhumed the flow from a valley fill and scoured its surface. The flood caused the draining of American Falls Lake, which was formed by the blocking of the Snake River by a lava flow from Cedar Butte about 72,000 + 14,000 years ago. Paleoenvironmental interpretations based on the microfossil assemblages in deposits in the American Falls area support the conclusion of Ridenour (1969) that the lower part of the American Falls Lake Beds of Trimble and Carr (1961b) is of alluvial origin. This fine-grained alluvium was probably deposited not long before 72,000 years ago. The alluvial gravel that underlies the fine-grained alluvium of the upper part of the American Falls Lake Beds and contains the remains ‘U. S. Geological Survey, Federal Center, Denver, Colorado 80225. of Bison latz~rons and other vertebrates may therefore also be not much older than 72,000 i 14,000 years, although its age may exceed 100,000 years.