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Pliocene and Pleistocene Volcanic Ash

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Petrographic Comparison of Pliocene and Pleistocene Volcanic Ash from Western Kansas

by Ada Swineford and John C. Frye

Originally published in 1946 as Kansas Geological Survey Bulletin 64, Part 1. This is, in general, the original text as published in 1946. The information has not been updated.


Lenticular, discontinuous deposits of volcanic ash occur in western Kansas in at least two stratigraphic positions within the Pliocene Ogallala formation and in one or more within the Pleistocene strata. To explore the feasibility of stratigraphic correlation by petrographic differences in the volcanic ash, we have studied 30 samples from 7 Pliocene localities and 34 samples from 1.7 Pleistocene localities. A color comparison showed a consistent difference. Chemical analyses showed a lower percentage of iron in the Pleistocene ash. The specific gravity of the Pleistocene ash is lower than that of the Pliocene ash. A refractive index of about 1.501 was common to the ash from all but two localities studied. Diagnostic characteristics of individual shards from the Pleistocene samples are numerous elongated vesicles, smaller radius of curvature, and greater abundance of rodlike fragments. Minor differences within the group suggest the occurrence of three ash falls within the Pliocene. All but two samples of Pleistocene ash are believed to represent a single fall.

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Kansas Geological Survey, Geology
Placed on web Aug. 10, 2007; originally published April 1946.
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