Ogallala Group or Formation

Silt, sand, sandstone, gravel and conglomerate. Predominantly interfingered fine- to coarse grained, poorly sorted, arkosic, fluvial deposits of light-gray, light-olive-gray, and grayish-green calcareous silt and sand, and locally poorly consolidated conglomerate, sandstone, and siltstone.
State Nebraska
Name Ogallala Group or Formation
Geologic age Miocene
Lithologic constituents
Major
Sedimentary > Clastic > Mixed-clastic > Siltstone-mudstone (Bed)
Sedimentary > Clastic > Mixed-clastic > Conglomerate-sandstone (Bed)
Incidental
Igneous > Volcanic (Pyroclastic-air fall)
Comments Lenses and tabular zones of carbonate-cemented silt and sand are common. In a few southeast localities, fine to coarse sand and fine gravel are cemented by yellowish-green opal or chaledony. Deposits contain some local beds of volcanic ash. Lithology varies laterally and vertically within a short distance. Includes in decending order: Ash Hollow and Valentine FMs. Approx. max thickness 650 ft. May contain some deposits of pre-Miocene age.
References

Burchett, R.R., 1986, Geologic Bedrock Map of Nebraska: Nebraska Geological Survey. Scale 1:1 million.

Diffendal, R.F. Jr, 1991, Geologic Map Showing Configuration of Bedrock Surface, North Platte 1x2 Quad. Nebraska: USGS Map I-2277. Scale 1:250K.

https://pubs.er.usgs.gov/publication/i2277

Swinehart, J.B., Souders, V.J., DeGraw, H.M., and Diffendal, R.F., 1985, Cenozoic paleography of the west-central United States, Society of Economic Paleontologist and Mineralogist, RMS, Rocky Mtn. Paleogeography Syp.3, p 209-229.

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