Zircon fission-track age of 0.45 million years on ash in the type section of the Merced Formation, west-central California

Abstract

A zircon fission-track age of 0.45 ± 0.08 million years (weighted average of 15 samples) has been determined on an ash interbedded with Pleistocene marine or littoral strata in the type section of the Merced Formation at two localities south of San Francisco. Separation of zircon crystals into several sample groups based on the presence or absence of glass coats, color, and surface texture has made it possible to identify the pyrogenic, detrital, and inferred accidental (xenocrystic) components in the zircon separates. The age obtained on pyrogenic zircons agrees closely with the youngest (0.45 ± 0.18 million years) of several K-Ar ages run previously on plagioclase and hornblende separates from the same localities. The older K-Ar ages (0.7 to 2.1 million years) on this ash are probably a result of detrital or accidental contamination, as determined by analyses on back-picked and untreated splits of the same feldspar separate.