Remanent Magnetization of Lower to Middle Eocene Basalt Flows from Oregon

Abstract

THE Siletz River Volcanic Series of north-western Oregon offers an ideal opportunity to determine the direction of the Earth's magnetic field in the western United States during the early Tertiary. The formation consists of 3,000–5,000 ft. of mainly submarine basaltic flows, flow breccia and tuff. Interbedded clastic sedimentary strata have yielded more than thirty species of fossils including brachiopoda and Turritella andersoni cf. subsp. susanae Merriam, on the basis of which J. Durham correlated this formation with the Capay shale of California¹. Additional faunal assemblages, including some foraminifera collected subsequently to those referred to above, confirm this correlation and clearly establish the age of the Siletz River Volcanic Series as early middle Eocene to lower Eocene. Petrographically, the flows are porphyritic in texture, with phenocrysts of plagioclase and augite in a ground mass which consists of volcanic glass, plagioclase laths, and granules of augito and magnetite, the latter constituting as much as 15 per cent of the rock¹.