Citation/Abstract

The upper Miocene Montesano Formation crops out in the southern Olympic Peninsula and unconformably overlies the middle Miocene Astoria(?) Fm. The Montesano Fm was deposited in a shallow embayment that received petrographically-distinctive sediment from sources in the newly-uplifted Olympic Peninsula and from sources in the Cascade volcanic arc. The detrital clay in the Montesano Fm is composed mainly of smectite, with (in decreasing order): discrete chlorite; [mixed layer/amorphous components]; illite; and kaolinite. The Olympic highlands are the probable sources for chlorite, kaolinite and some of the mixed-layer/amorphous components in the Montesano clay. The Montesano Fm experienced a low-temperature low-pressure clinoptilolite-smectite facies diagenesis (not exceeding 50-60 degrees C). An allodesmine pinniped and a Mysticeti are the first vertebrates reported from the formation. The Astoria(?) Fm sediment had no Olympic sources. The Astoria(?) clay is a purer smectite with a smaller percentage of [mixed-layered/amorphous components] and only traces of illite and [kaolinite + discrete chlorite].