Volcanic ash occurs in the Great Plains of Kansas and adjacent states at several stratigraphic positions within the Pliocene and Pleistocene deposits. Although at least two different horizons of ash, which may represent three ash falls, are known from the Pliocene, and one or more, possibly representing as many as three ash falls, may occur within the Pleistocene, they can be placed in two natural groups--one of about mid-Pliocene age and the other of about mid-Pleistocene age.
Several characteristics are common to all Pleistocene samples, which serve to differentiate them from all Pliocene samples. The Pliocene ash is typified by a neutral gray color; thin, transparent, slightly curved platy shards which are relatively free from vesicles; a specific gravity of 2.33 or more; and a ferric oxide content generally more than 2 per cent. The Pleistocene ash is lighter in color; the shards are more sharply curved and include abundant elongate vesicles, commonly arranged in clusters. Rodlike or fibrous shards were found in samples from all Pleistocene localities.
Within the group of Pliocene samples, differences in refractive index of the shards from certain localities suggest that these samples have been derived from three ash falls; however, one of these localities (Hemphill County, Texas) is so remote from the others that differences in the ash alone cannot prove a difference in age. The high index samples from Norton County (nos. 26-29) have not been shown by fossil seeds to be in a different stratigraphic position than the large group of samples from that area (nos. 1-21), whereas no petrographic differences were found between Pliocene ash deposits in Norton County believed, on the evidence of fossil seeds, to be at slightly different positions within the Ogallala.
There may be as many as three ash falls within the Pleistocene strata. One is widespread and is represented by samples no. 31 to 45 and 47 to 61 from Ellsworth, Jewell, Lincoln, McPherson, Meade, Ness, Ottawa, Seward, Sheridan, and Smith Counties, in central and western Kansas; by sample no. 63 from Frontier County, Nebraska; and by sample no. 64 from Beaver County, Oklahoma. Another fall is suggested by an upper ash deposit in McPherson County (no. 46) which has a slightly different refractive index, and still another may be represented by the sample from Webster County, Nebraska (no. 62), which contains numerous flakes of biotite.
Kansas Geological Survey, Geology
Placed on web Aug. 10, 2007; originally published April 1946.
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