Cosmogenic 3he helium age dating
The base of the lava flow, thus, the top of the gravels, is about 100 feet above present-day river level, and for several decades, geologists have used the position and age of this dam to say that, after initial incision of the Grand Canyon began, the river downcut to within 100 feet of its current level by 1.2 million years ago (Hamblin, 1994). Displacement rates on the Toroweap and Hurricane faults: Implications for Quaternary downcutting in Grand Canyon.
The active Toroweap fault runs north-south across the river, offsetting by different amounts lava flows, river gravel, and hillslope deposits of different ages, including the Toroweap and Prospect Dams (Figures 1 and 2 respectively).
Cosmogenic ages of lava flows on the Uinkaret Plateau range in age from 1,000 to 400,000 years and for the most part, agree with Ar/Ar ages of the same flows.
In some cases, the K-Ar ages agree with the cosmogenic ages, as well, however, the K-Ar ages are usually two to three times older than ages provided by the other methods. E., Geochemical discrimination of five Pleistocene lava-dam outburst-flood deposits, western Grand Canyon, AZ, Journal of Geology, v.
Radioactive potassium is a small part of all the potassium that make up minerals within western Grand Canyon lavas.