The Chalk Butte Formation in Oregon is part of the Idaho Group, a regional sequence of sedimentary and volcanic-related rocks, and is notable for preserving fossils from the Neogene period. Proposed as one division of Idaho Group rocks in the Mitchell Butte Quadrangle, it was identified alongside the Grassy Mountain Basalt Formation and the Kern Basin Formation. Its rocks consist mainly of tuffaceous sandstones, siltstones, and conglomerates, with lesser tuff, ash beds, and freshwater limestone, reflecting sediment deposition influenced by volcanic ash. At its type location, a partial measured section recorded it at about 538 feet thick. It is important for documenting southeastern Oregon’s Neogene environments and prehistoric life.