Dalles Formation

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
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 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.

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.

See also

• List of fossiliferous stratigraphic units in Oregon
• Paleontology in Oregon