Volcanic Ashes: Time Markers in Soil Parent Materials of the Southern Plains

Volcanic ashes represent geologic time lines or markers in soil parent materials. The position in the landscape of the ash deposit is important to understanding the development of the landscape. The location of an ash layer in soil parent material facilitates understanding soil genesis. The objective of this study was to date, characterize, and correlate ashes found in unconsolidated soil parent materials of the Southern Plains. Ages of the ashes were determined by fission-track dating of glass shards from the deposits. Major-element content (g kg⁻¹ Ca, Fe, K, Na, and Ti) of glass shards was determined by inductively coupled argon plasma (ICAP) spectrophotometry. Trace-element composition (mg kg⁻¹ Zn, Zr, Nb, Y, Rb, Sr, Th, and Pb) of shards was measured with x-ray fluorescence (XRF). Shard morphology and index of refraction were measured with scanning electron and petrographic microscopes. Forty-six ash deposits were dated. Shard ages ranged from 0.53 m.a. (million years ago) to 9.95 m.a. Forty-two of the 46 ashes had measured ages of <2.2 m.a. Elemental compositions of the ashes were similar to previously reported compositions of other ashes found in the western USA. Cluster analysis of the age and characterization data grouped the ashes into one large and several smaller groups. Thirteen ashes were correlated with the Lava Creek B ashfall, three with the Mesa Falls ashfall, and two with the Huckleberry Ridge ashfall from the Yellowstone Plateau volcanic field of northwestern Wyoming and eastern Idaho. Five ashes were correlated with the Tsankawi ashfall and one ash correlated with the Guaje ashfall from the Jemez volcanic field of central New Mexico. Two ashes were correlated with the Bishop ashfall of the Long Valley caldera of eastern California. Ratios of Y to Rb, Sr to Th, and Sr to Pb, along with glass shard age, were identified using discriminant function analysis as the most important characteristics in grouping ashes.