Field, petrographic, geochemical, and limited chronologic information allow for construction of a composite stratigraphic section for the approximately 1-km-thick exposure of basaltic lavas in the Pueblo Mountains region of the Oregon Plateau. Comparison of the Pueblo basalts with those of Steens Mountain and the Columbia Plateau suggests that these flood-basalts were derived from a common mid-Miocene magmatic event, possibly marking the initiation of back-arc spreading along the western margin of the Wyoming craton.

Geochemical and Sr-isotopic variations within the Pueblo basalts and, more specifically, vertical variations within the Pueblo composite section, define two distinct groups of basalt and three phases of evolution. Within-group variations are modeled by low-pressure fractional crystallization, whereas the between-group variations are attributed primarily to different degrees of interaction between crustal materials and ascending Pueblo primary magmas during the three distinct phases of volcanism.