MAGNETOSTRATIGRAPHY OF THE IMNAHA AND GRANDE-RONDE BASALTS IN THE SOUTHEAST PART OF THE COLUMBIA PLATEAU

Two hundred and thirty-one oriented samples were collected from basalt flows at Dug Bar (OR), Fall Creek (OR), Grande Ronde (WA), Joseph Creek (WA), and Rocky Canyon (ID), the stratigraphy of which has been established by a detailed field and petrochemical study. The stable remanent magnetism of the samples, when compared to the magnetic polarities determined independently using a portable fluxgate magnetometer confirm the fluxgate magnetometer polarity results in all cases but clarify the presence of flows whose polarity is transitional, with low latitutde virtual magnetic poles. The results demonstrate that the Imnaha Basalt belongs to a single normal polarity epoch (No), except for a few basal flows with reversed polarity (Ro) and the top few flows, which have a transitional polarity. The abrupt chemical change from Imnaha to Grande Ronde Basalt occurred during the N0/r1 polarity transition, with the earliest two or three flows of Grande Ronde Basalt possessing transitional polarity and the subsequent flows reversed (R1), normal (N1), and then reversed (R2) polarities.-Authors