Dalles Formation
Geologic formation in Oregon, United States
From Wikipedia, the free encyclopedia
The Dalles Formation is a geologic formation in Oregon, mapped in the vicinity of The Dalles and adjacent Columbia River region. It consists chiefly of weakly consolidated Miocene to lower Pliocene fluvial and volcaniclastic deposits that overlie the Columbia River Basalt Group.[1]
| Dalles Formation | |
|---|---|
| Stratigraphic range: Neogene (Miocene to possibly early Pliocene) | |
| Type | Formation |
| Unit of | Columbia River Basin |
| Sub-units | Informal lower coarse conglomeratic beds; upper finer tuffaceous sandstone and siltstone |
| Underlies | Local surficial deposits (Quaternary alluvium and terrace gravels) |
| Overlies | Columbia River Basalt Group (Yakima Basalt) |
| Thickness | Variable; locally several hundred feet (rapid lateral variation reported) |
| Lithology | |
| Primary | sandstone, siltstone, conglomerate |
| Other | tuff, tuffaceous sandstone, volcanic breccia, clayey volcanic detritus |
| Location | |
| Coordinates | 45.408°N 121.336°W |
| Region | Wasco County, Oregon |
| Country | United States |
| Extent | Intermontane Belt and valley margins near The Dalles |
| Type section | |
| Named for | The Dalles |
| Named by | Arthur M. Piper (1932) |
The formation is composed of interbedded sandstone, shale, conglomerate, and tuffaceous materials derived from both stream deposition and volcanic activity. Local coarse conglomeratic zones contain angular volcanic fragments, while finer tuffaceous sandstones and silts are more widespread but discontinuous. The formation also preserves Neogene fossil material consistent with its (Miocene–Pliocene) age assignment.
