COMPARATIVE GEOCHEMISTRY OF UNKNOWN BASALTS FROM PAULINA, OR TO DETERMINE MAP UNITS

The Crooked River region near Paulina, Oregon has exposures of Oligocene to Miocene volcanic rocks. Generally, the lowest exposed formation is the Clarno, which has various lithologies. Present above is the John Day formation and Mascall Formation of tuffs and tuffaceous sandstones, the Rattlesnake formation of siliciclastics and the prominent Rattlesnake ash-flow tuff. Above this sequence is the Columbia River Flood Basalts (CRBs) that erupted 18-8 Ma. During the Neogene, all of these formations have undergone significant deformation including both broad scale folding and faulting. Recent geologic mapping in the Paulina area has revealed complex structure that distorts the volcanic package. In some locations, it becomes difficult to be sure if basalt outcrops are of the lower Clarno Formation or the upper CRB Formation. This study aims to compare the geochemistry of the unknown basalts to complied suites of basalts from known formations in order to determine the proper, or confirm the suspected, map unit.

Elemental abundances were determined using X-ray Fluorescence Mass Spectrometry (XRF), from which minor and major elements are reported. A literature search was conducted to create a database of published Clarno and CRB geochemistries, much of which exists for the CRB group but relatively little data has been published on the Clarno. Of the unknown basalts, we have determined that many of them are more geochemically similar to the CRBs, while few of them are similar to the Clarno. These results support the field determination of the units, which confirms the presence of large, inactive normal faults previously unrecognized.