| Klondike Mountain Formation | |
|---|---|
| Stratigraphic range: Eocene (Ypresian) | |
 Klondike Mountain Formation outcrop at site A0307 | |
| Type | Geological formation |
| Unit of | Okanagan Highlands |
| Sub-units | Tom Thumb Tuff, Lower & Upper Members |
| Underlies | Glacial deposits |
| Overlies | Sanpoil Volcanics & others |
| Thickness | up to 3,200 ft (980 m) |
| Lithology | |
| Primary | Variable |
| Other | See text |
| Location | |
| Coordinates | 48°42′N 118°42′W / 48.7°N 118.7°W |
| Approximate paleocoordinates | 54°24′N 101°54′W / 54.4°N 101.9°W |
| Region | Okanagan highlands |
| Country |  United States |
| Extent | Okanagan highlands |
| Type section | |
| Named for | Klondike Mountain |
| Named by | Muessig |
| Year defined | 1962 |
  Klondike Mountain Formation (the United States)  Klondike Mountain Formation (Washington (state)) | |
The Klondike Mountain Formation is an Early Eocene (Ypresian) geological formation located in the northeast central area of Washington state. The formation is composed of volcanic rocks in the upper unit and volcanic plus lacustrine (lakebed) sedimentation in the lower unit. the formation is named for the type location designated in 1962, Klondike Mountain northeast of Republic, Washington. The formation is a lagerstätte with exceptionally well-preserved plant and insect fossils has been found, along with fossil epithermal hot springs.
The Klondike Mountain formation is the youngest in a series of formations which belong to the Challis Sequence volcanics, and is the southernmost of the Eocene Okanagan Highlands paleolake lagerstätten. The formation unconformably overlies rocks of the Eocene Sanpoil Volcanics and much older Triassic and Permian formations. The formation is bounded on its edges by a series of high-angle strike slip faults, which have contained the Klondike Mountain Formation in a series of graben structures, such as the Republic Graben. Public access to a fossiliferous outcrop at the north end of Republic is mediated by the nonprofit Stonerose Interpretive Center.