Ohanapecosh Formation
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| 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 volcaniclastic 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 Mount 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 Mount Rainier and Lake Tapps, east to Little Naches River valley.[1]
The contact of the Ohanapecosh Formation on the Puget Group is everywhere conformable and the Spiketon Formation and Renton Formation reflect a continuous process without a break in time. In contrast, the contact with the Naches Formation is an unconformity as seen in the Summit Creek Sandstone (~43 to 37 Ma) in the areas from White Pass east to the Naches River.[1] In the Mount Rainier National Park area, the Ohanapecosh Formation is overlain by the Oligocene (25-27 Ma) Stevens Ridge Member, that is the lower part of the Fifes Peaks Formation. At Backbone Ridge, southeast of Mount Rainier, clasts of the Ohanapecosh Formation and tree trunks are found in the base of the lowest Stevens Ridge Member.[1]
Various late Oligocene and Miocene eruptive centers are preserved in central Washington. The Mount Aix caldera (late Oligocene), Tieton volcano (Miocene) and Columbia River Basalt Group (Miocene) are late Oligocene and Miocene eruptive centers east of the Mount Rainier National Park. Fifes Peaks volcano is to the northeast of the national park, and the Tatoosh pluton is to the south.[1] The Eocene-Miocene formations are covered by thick Quaternary volcaniclastic deposits and lavas.