Potassium–Argon Dating of Plio-Pleistocene Intrusive Rocks

Abstract

THE potential of the potassium–argon technique in the dating of young rocks can be evaluated by applying it to intrusive igneous rocks which are well dated by conventional geological methods. Because of their possible interest, we are reporting the dating of two such igneous rocks. They occur at Sutter Buttes in the Sacramento Valley, forty miles north of Sacramento, California. Rhyolite plugs of Plio-Pleistocene age intrude and upturn sedimentary formations of late Cretaceous to early Pliocene age. Erosion uncovered the rhyolite plugs before the ensuing period of andesitic intrusions and extrusions, suggesting that the entire igneous cycle covered many thousands of years. Canyons 1,500 ft. deep have since been incised in the sedimentary complex, and much of the volcanic debris which once covered the area to a depth of 500 ft. or more has been stripped by erosion during the time since igneous activity ceased. One of the rhyolite plugs and the youngest andesite block-lava flow were dated. Biotite was used for the age determinations, essentially pure concentrates being obtained by bromoform and diiodomethane methods. Tyler mesh sizes were −35 + 150. A branching ratio of 0.110 and a decay constant of 0.558 × 10−9 gm. were used. The pertinent age data are given in Table. 1.