The volcaniclastic Ohanapecosh Formation is an early state of Cascade volcanism. It has been dated to the middle

Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period that extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that defin ...

6 to 28 MaMartin Jutzeler, Characteristics and origin of subaqueous pumice-rich pyroclastic facies: Ohanapecosh Formation (USA) and Dogashima Formation (Japan); University of Tasmania, Australia, April 2012 The strata are as much as thick, with exposures visible in more than of a total area exceeding . It is found throughout

Mount Rainier National Park Mount Rainier National Park is a List of national parks of the United States, national park of the United States located in southeast Pierce County, Washington, Pierce County and northeast Lewis County, Washington, Lewis County in Washington (sta ...

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 The Columbia River Gorge is a canyon of the Columbia River in the Pacific Northwest of the United States. Up to deep, the canyon stretches for over as the river winds westward through the Cascade Range, forming the boundary between the state ...

, including

Mount St. Helens Mount St. Helens (known as Lawetlat'la to the local Cowlitz people, and Loowit or Louwala-Clough to the Klickitat) is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States ...

and Mount Adams. It extends from the west at Mount Rainier and

Lake Tapps Lake Tapps is a reservoir in Pierce County, Washington. It was created in 1911 by Puget Sound Power & Light and operated for hydroelectric power until it ceased power production in 2004. The reservoir was sold to the Cascade Water Alliance, a col ...

, east to Little Naches River valley.

Description

The contact of the Ohanapecosh Formation on the

Puget Group The Puget Group is a Group (geology), geologic group in Washington (state). It preserves fossils dating back to the Paleogene Period (geology), period. In the northern area of group around Renton, Washington, the Group has been divided into thr ...

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 An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...

as seen in the Summit Creek Sandstone (~43 to 37 Ma) in the areas from White Pass east to the Naches River. In the Mount Rainier National Park area, the Ohanapecosh Formation is overlain by the

(25-27 Ma) Stevens Ridge Member, that is the lower part of the

Fifes Peaks Formation Description Fifes Peaks Formation consists of flows that are vesicular, basaltic andesite. Weathering the rock creates shades of dark brown. The newly fractured blocks are medium- to dark-gray colors. Some flows are fine grained, most are porp ...

. 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. Various late Oligocene and Miocene eruptive centers are preserved in central Washington. The

Mount Aix Mount Aix is a mountain summit in Yakima County of Washington state. Description Mount Aix is set in the William O. Douglas Wilderness on land administered by the Wenatchee National Forest and is the highest point in the wilderness area. Mo ...

caldera (late Oligocene), Tieton volcano (Miocene) and

Columbia River Basalt Group The Columbia River Basalt Group (CRBG) is the youngest, smallest and one of the best-preserved continental flood basalt provinces on Earth, covering over mainly eastern Oregon and Washington, western Idaho, and part of northern Nevada. The b ...

(Miocene) are late

and

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

eruptive centers east of the Mount Rainier National Park.

Fifes Peaks Fifes Peaks is a compact group of summits and spires from an eroded volcanic cone located in Yakima County, Washington, Yakima County in Washington (U.S. state), Washington state. Fifes Peaks is situated in the Cascade Range on the east side of ...

volcano is to the northeast of the national park, and the Tatoosh pluton is to the south. The Eocene-Miocene formations are covered by thick Quaternary volcaniclastic deposits and lavas.

Lithology

The Ohanapecosh Formation is composed of volcanic clastic rocks and lava flows. The lava flows are interbedded with the coarse volcanic clastic rocks which are mostly mudflow deposits. Locally they can be thick, grading into finer clastic rocks. The ash flows and rhyolite flows are a small portion, less than 1 percent, of the formation. The Ohanapecosh Formation has been divided into three parts: (1) lava flow mudflow complexes, (2) adjacent accumulations of volcanic clastic rocks, and (3) ash flows and rhyolites. Fiske, Richard S., Clifford A. Hopson, and Aaron C. Waters; Geology of Mount Rainier National Park Washington; Geological Survey Professional Paper 444; United States Government Printing Office, Washington, D.C.; 1963

Lava flow-mudflow complexes

Two major complexes of lava flows and associated mudflow deposits have been identified. The smaller one is in an irregular north-trending belt east and southeast of Mount Rainier. It is best exposed in the steep cirque walls of the Sarvent Glacier, i.e.

Banshee Peak Banshee Peak is a mountain summit located in Mount Rainier National Park in Pierce County of Washington state, US. As part of the Cascade Range, Banshee Peak is situated west of the Cowlitz Chimneys and east of Panhandle Gap, with the Sarvant ...

. The larger lava complex is in the Mount Wow-Satulick Mountain area, the southwestern part of the park. It can also be found in the valley of the North

Puyallup River The Puyallup River ( ) is a river in the U.S. state of Washington. About long, it is formed by glaciers on the west side of Mount Rainier. It flows generally northwest, emptying into Commencement Bay, part of Puget Sound. The river and its t ...

on the western park boundary. The Ohanapecosh formation of these areas is separated by extensive outcrops of the Tatoosh pluton and by deposits from the Mount Rainier volcano. The lava flows and interbedded mudflows are seen on Stevens Peak. These complexes are lens shaped. The maximum thickness of the Sarvent complex is about , and the Mount Wow complex is at least . Lava flows and coarse mudflow deposits form more than 70 percent of these units. Fresh exposures of Ohanapecosh lava are dark brownish gray, greenish gray, or maroon; most weathered surfaces are brown or maroon. Individual flows are from thick. All the Ohanapecosh lavas have been pervasively altered.

References

{{reflist, 3 Geologic formations of Washington (state) Miocene Series of North America Messinian Fluvial deposits Pierce County, Washington Lewis County, Washington Yakima County, Washington Rhyolite formations