Correlation of the Bishop Ash, a Pleistocene Marker Bed, Using Instrumental Neutron Activation Analysis

Abstract

ABSTRACT Neutron activation analyses confirmed a previous correlation of a white rhyolitic volcanic ash bed with the Bishop Tuff of eastern California. Distinctions were made between the glass fractions of Bishop and Pearlette-like ashes, both of which occur in middle Pleistocene deposits of California, Utah, Colorado, and Nebraska. A third type of ash (ash of Green Mountain Reservoir), petrographically similar to the Bishop Tuff, appeared to be chemically distinct. A coefficient was introduced as a simple measure of multivariate similarity. Discriminant function analysis indicated that Mn and Sm were most useful in distinguishing chemically between Bishop, Green Mountain Reservoir, and Pearlette-like ashes. Both statistical methods confirmed conclusions drawn from visual observation of the da a.