THE SIGNIFICANCE OF GEOCHEMISTRY AND ERUPTIVE TIMING IN EXTENDING SPATIAL DISTRIBUTION AND EVALUATING STRATIGRAPHIC CORRELATION OF THE PICTURE GORGE BASALT

The Picture Gorge Basalt (PGB) of the Columbia River Basalt Group (CRBG) has been limited in its eruptive volume (<3,000 km³) covering ~10,000 km² and representing only 1.1 % of the main phase CRBG. New age data reveal that the PGB is the earliest and longest eruptive unit among other main-phase CRBG formations, and that some dated flows reach ~100 km beyond the previously mapped extent. This study reassesses the spatial distribution, eruptive volume, and stratigraphy of the PGB. Field observations coupled with geochemical data indicate that extensive outcrops of unstudied basaltic lavas and dikes south of the type locality at Picture Gorge and elsewhere correlate with PBG. Over 100 km southeast of Picture Gorge, basaltic lavas in Malheur Gorge previously assigned to other CRBG units, exhibit geochemical signatures most similar to the PGB.

Stratigraphic correlation of the PGB is facilitated by comparison of 20 major and trace elemental abundances via a principal component analysis and age constraints from underlying/overlying units. Geochemical identifiers to differentiate PGB from other main-phase CRBG formations include lower TiO2 (< 2 wt. %) concentrations, lower incompatible trace elements (i.e. La, Th, and Y) abundances at a given SiO₂ content, and relative enrichment in large-ion-lithophile elements (LILE) on a mantle-normalized trace element diagram. Our work leads to a new distribution area of PGB that is more than double the original distribution and a new minimum eruptive volume estimate of ~4,200 km³.

Geochemical characteristics of PGB point to a magmatic source component distinct from the other main-phase CRBG units. This source must be extensive as there are lava flows and dikes with compositions alike PGB extending as far southeast as Lake Owyhee, as far southwest as Hart Mountain, at Dooley Mtn. in the north, and there are also found at the base of the Imnaha formation. Finally, ages indicate that basalts originating from this source erupted early to the tail end of main phase CRBG activity, from 17.23 Ma to 15.76 Ma. Although within this period an eruptive hiatus of ~400k is suggested from PGB ages and geochemistry, as lavas and dikes of the extended distribution mostly correlate with the earlier and later stratigraphic sections at Picture Gorge.