Thermoluminescence of Limestones of Mississippian Madison Group in Montana and Utah

Abstract

Glow curves of limestone samples from four sections of the Madison group were run in an effort to determine the extent to which thermoluminescence could be used to subdivide the Madison into smaller stratigraphic units which could be used in correlation. It was found possible to differentiate the Lodgepole and the Mission Canyon formations of the Madison group on the basis of the shapes of their glow curves, but further subdivision of these units was not successful. Natural radioactivity of the rocks and their impurity content have been considered important factors in determining the intensity of thermoluminescence and the shape of the glow curve. The studies reported here failed to verify the role of radioactivity, but the magnesium content of the limestone was shown to have a profound effect on the shape of the glow curve. Calcitic limestones tend to have a high low-temperature peak in thermoluminescence; whereas magnesian limestones and dolomites have a high middle-temperature peak. The discrimination between the Lodgepole and Mission Canyon formations of the Madison group was made primarily on the basis of the relative heights of the low- and middle-temperature peaks. This relation depends on the amount of magnesium in the limestone. Thus the distinction between the two formations by thermoluminescence is largely based on the difference in their magnesium content.