Rock-varnish coatings on cobbles from geomorphic surfaces and exposed deposits in arid environments are an effective medium for dating over a time range of several thousand to a few million years. A new analytical method for dating of rock varnish is presented wherein the varnish cation ratio (VCR) is determined by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray analyzer (EDAX).
The experimental SEM method is a nondestructive technique that has several potential advantages over the original method of analysis, described by R. I. Dorn, that uses particle-induced X-ray emission (PIXE) of varnish scraped from rock surfaces. The SEM method can potentially eliminate analytical errors due to contamination from rock substrate because variations in varnish thickness and irregularities on the substrate surface are examined before cation ratios are determined. Because varnish surfaces remain intact, varnish sites that yield anomalous results may be reanalyzed or verified. In addition, the general accessibility of scanning electron microscopes will make rock-varnish dating more widely available for use in Quaternary studies.
Cation ratios were calculated for rock varnish from Española Basin, New Mexico, and the Yucca Mountain region, Nevada, and were used to construct rock-varnish dating curves for these areas.