The Picture Gorge Basalt Subgroup was mapped over an area of 1,600 km² in the John Day Basin in north-central Oregon. Lateral continuity of the Twickenham, Monument Mountain, and Dayville Basalts is demonstrated, and each is elevated to formation status within the Picture Gorge Basalt Subgroup of the Columbia River Basalt Group. Seventeen members are defined based on field and compositional characteristics. Sixty-one flows are identified, of which 39 belong to recognized members. The remaining 22 flows that are unassigned are of limited areal extent. Compositional data and field characteristics were used with the assistance of discriminant analysis to extend correlations of members beyond their type localities.

The earliest Twickenham Basalt flows filled areas of moderate relief in the John Day Basin. The resultant flat topography was covered by numerous extensive (and some less extensive) flows of the Monument Mountain and Dayville Basalts, creating a crude shield-like structure in east–west profile. Poor correlation of the Picture Gorge type section with sections farther north suggests at least two loci of eruptions within the John Day Basin, one centered near Picture Gorge, the other between Kimberly and Monument.

Compositional variation within flows of Twickenham Basalt is commonly great, due to in situ fractionation. Monument Mountain Basalt flows are comparatively homogeneous, and the formation as a whole displays minor compositional variation. Flows within the Dayville Basalt are also homogeneous, and the formation is characterized by an abundance of areally restricted and compositionally diverse flows.