EVALUATION OF POTENTIAL SOURCE VENTS FOR THE GRAND FALLS LAVA FLOW, SAN FRANCISCO VOLCANIC FIELD, NORTHERN ARIZONA

The San Francisco Volcanic Field lies along the southern margin of the Colorado Plateau in northern Arizona. It is host to over 600 volcanic vents ranging in age from 6 Ma in the western portion of the field to approximately 900 a in the eastern region. The 20,000 a Grand Falls lava flow, located near the southeastern margin of the field, flowed toward the northeast for ~10 km where it spilled into the 65 m deep Little Colorado River canyon forming a dam. The flow continued approximately 15 km upstream and 25 km downstream. Younger lava flows overlie the southeastern end of the flow making a direct field correlation to the source vent impossible.

In much of the lay literature, the Grand Falls flow has been attributed to the eruption of Merriam Crater, a prominent cinder cone to the southwest of the flow. Because no data are available to substantiate this source, five potential vents including Merriam Crater and two associated vents, The Sproul, and Vent 3028 were evaluated as potential sources.

Basalt from Merriam Crater and The Sproul are alkali olivine basalt whereas flows from Vent 3028 are basanite. Compositionally, the Grand Falls Flow lies between the two, straddling the basanite – alkali olivine basalt boundary. Texturally, all of these basalts are holocrystalline to hypocrystalline and porphyritic aphanitic. Olivine is the dominant phenocryst in all of the flows except Vent 3028, in which clinopyroxene almost exclusively present. Mineral compositions from all of these vents exhibit considerable overlap, thus cannot be used to evaluate potential source vents.

Whole rock analyses show that Merriam Crater vents are slightly higher in silica relative to the Grand Falls Flow. Additionally, the Grand Falls flow, Vent 3028, and the Sproul are enriched in K₂O/Th and Zr/Y and depleted in La/Ta and Th/Ta relative to Merriam vents.

These geochemical variations, coupled with nonlinear trends on variation diagrams, preclude attributing the flow to an eruption of Merriam Crater or the associated vents. Field evidence suggests that Vent 3028 produced flows that continued further to the north, and never reached the Little Colorado River, thus it is also not a likely candidate. Thus, The Sproul is the only reasonable contender as source vent for the Grand Falls flow.