| Ohanapecosh Formation | |
|---|---|
| Stratigraphic range: Priabonian ~ | |
| Type | Geological formation |
| Sub-units | Chinook Pass association, White Pass association, Johnson Creek association |
| Underlies | Pleistocene terrace deposits |
| Overlies | Puget Group, Spiketon Formation, Renton Formation, & Naches Formation |
| Area | >400 square kilometres (99,000 acres) |
| Thickness | 9,000–10,000 m (30,000–33,000 ft) |
| Lithology | |
| Primary | Pyroclastic flow, Rhyolite |
| Other | volcanic Clastic rocks, pumice, lapilli, Phenocrysts, plagioclase |
| Location | |
| Coordinates | 28°36′N 97°42′W / 28.6°N 97.7°W |
| Approximate paleocoordinates | 29°00′N 95°18′W / 29.0°N 95.3°W |
| Region | Mount Rainier National Park, Lewis, & Yakima counties, Washington |
| Country |  United States |
| Extent | White Pass (Washington) |
| Type section | |
| Named for | Ohanapecosh Hotsprings |
| Named by | Ingram |
| Year defined | 1954 |
  Ohanapecosh Formation (the United States)  Ohanapecosh Formation (Washington (state)) | |
The volcanic clastic Ohanapecosh Formation is an early state of cascade volcanism. It has been dated to the middle Oligocene [36 to 28 Ma].[1] The strata are as much as 3 kilometres (1.9 miles) thick, with exposures visible in more than 400 square kilometres (99,000 acres) of a total area exceeding 700 square kilometres (170,000 acres). It is found throughout Mount Rainier National Park and the surroundings mountains. It is the foundation on which the Mt Rainier volcano was built. The formation has been identified in the Snoqualmie area to the north and as far south as the Columbia River Gorge including Mount St. Helens and Mount Adams. It extends from the west at Mt Rainier and Lake Tapps east to Little Naches River valley.[1]