- GeoRef, Copyright 2004, American Geological Institute. Reference includes data from Bibliography and Index of North American Geology, U. S. Geological Survey, Reston, VA, United States
A review of bulk compositional changes in phenocryst minerals and rock fragments of volcanic origin in the volcanic sedimentary rocks of Tertiary age in Wyoming indicates that the cyclic sequence of volcanic eruptions in the Yellowstone-Absaroka Volcanic Field is reflected to some degree in the composition of volcanic sedimentary rocks of the basins. Rocks of late early Eocene age, upper Eocene age, early lower Oligocene age, middle Miocene age, and Pliocene age have consistent bulk compositions. This suggests that petrographic studies of volcanic sedimentary rocks may be useful in stratigraphic correlation.