Date of Award
Master of Science (MS)
Michael E. Nelson
The Calvert ash bed, Ogallala Group (Miocene), was examined using three geologic subdisciplines, tephrochronology, geochemistry, and stratigraphy, to identify distinctive characteristics that would allow accurate correlation of the Calvert ash and establish the Calvert as a time-stratigraphic marker. To determine these characteristics, Calvert ash collected at its type section, the Calvert Ash Mine, was the primary focus of this report. The Calvert is a light gray (N7), coarse ash containing both pyrogenic and xenocrystic minerals. These minerals include quartz, alkali feldspar (sanidine), plagioclase feldspar (oligoclase), magnetite, biotite, hematite, hornblende, zircon, clinopyroxene, apatite, and calcite. The shards are predominantly thin, iridescent, and platy, range in size from 1.41 to 0.09 mm, have an average refractive index of 1.502, and lack vesicles and inclusions. The Calvert ash was fission-track dated at 11.5 ± 0.48 Ma. Three bulk samples from the Calvert Ash Mine were analyzed by atomic absorption for major oxides and trace elements. Based upon the chemical concentrations of SiO2, Na2O, and K2O in these samples, and the IUGS classification of volcanic rocks lacking modal analysis, the Calvert is classified as a rhyolite. Optical characteristics determined by this study that are useful in distinguishing the Calvert from other Ogallala ashes found in Kansas are shard habit, shard size, and rare occurrence of vesicles. The chemical data (major oxides and trace elements) determined for the Calvert in this study cannot be considered distinctive due to alteration by hydration and groundwater leaching. Any correlations between the Calvert and other ashes based solely upon optical properties or chemical concentrations determined by this study are not reliable. Further study on the chemical composition of the Calvert is needed for accurate correlation. This study has not established the Calvert as a time-stratigraphic marker.
Copyright 1991 Sandra Potter
Potter, Sandra L., "Geologic Characteristics of the Calvert Ash Bed, Ogallala Group (Miocene), Western Kansas" (1991). Master's Theses. 2294.