Touchet Formation

Touchet Formation
Stratigraphic range: Late Pleistocene (Rancholabrean)
~0.0185–0.0158 Ma
↓
Touchet beds in Burlingame Canyon (the "Little Grand Canyon") near Lowden in the Walla Walla valley. Note distinct layers.
TypeGeological formation
UnderliesLoess
OverliesColumbia River Basalt Group
Area7,000 square miles (18,000 km2)
Thicknessup to 330 feet (100 m)[1]
Lithology
PrimarySediment[specify]
OtherBasalt, loess
Location
CoordinatesWalla Walla Basin
46°15′49″N 120°31′00″W / 46.26361°N 120.51667°W / 46.26361; -120.51667
Touchet Basin
46°06′00″N 118°39′00″W / 46.10000°N 118.65000°W / 46.10000; -118.65000
White Bluffs
46°42′25″N 119°27′23″W / 46.70694°N 119.45639°W / 46.70694; -119.45639
Yakima Basin
46°20′30″N 119°27′23″W / 46.34167°N 119.45639°W / 46.34167; -119.45639
RegionWashington, Oregon
Country United States
ExtentColumbia Basin of
Western North America
Type section
Named forTouchet, Washington
Named byFoster Flint
Year defined1938[1]

Map showing the elevations of the Mid-Columbia region. The lowland areas in brown routinely were flooded during the Missoula Floods, creating lakes.

The Touchet Formation or Touchet beds consist of well-bedded, coarse to fine sand and silt which overlays local bedrock composed of Neogene basalt of the Columbia River Basalt Group in south-central Washington and north-central Oregon. The beds consist of more than 40 to 62 distinct rhythmites – horizontal layers of sediment, each clearly demarcated from the layer below. These Touchet beds are often covered by windblown loess which were deposited later; the number of layers varies with location.[2][3][4] The beds vary in thickness from 330 ft (100 m) at lower elevations where a number of layers can be found to a few extremely thin layers at the maximum elevation where they are observed (1,150 ft (350 m)).[1]

The Touchet beds are one element in a chain of evidence which helped identify and define the progression of the Missoula Floods, which occurred around 18,400 to 15,700 calendar years ago.[4][5] During the floods, flow through the Wallula Gap was slow enough such that water pooled in a temporary lake, Lake Lewis. Lake Lewis back-flooded up the Yakima, Walla Walla, Touchet and Tucannon River Valleys. In these relatively calm arms of the lake, the slack waters deposited the suspended materials eroded from the scabland regions north of Lake Lewis, and redeposited them in pronounced layers before receding.[2]

  1. ^ a b c Cite error: The named reference Baker1978 was invoked but never defined (see the help page).
  2. ^ a b Cite error: The named reference Bjornstad was invoked but never defined (see the help page).
  3. ^ Cite error: The named reference MacEachernOthers2013 was invoked but never defined (see the help page).
  4. ^ a b Cite error: The named reference Waitt85 was invoked but never defined (see the help page).
  5. ^ Cite error: The named reference Lopes was invoked but never defined (see the help page).