CORRELATION OF LAVA FLOWS AT WUPATKI NATIONAL MONUMENT, NORTHERN ARIZONA

Wupatki National Monument, located on the southern margin of the Colorado Plateau in northern Arizona, is perhaps best known as an archaeological site with spectacular puebloan ruins. Throughout the Monument, excellent exposures of Permian Kaibab and Triassic Moenkopi Formations dominate the landscape with basalt capped mesas rising up to 60 meters high. These flows are part of the late Cenozoic San Francisco Volcanic Field (SFVF) which began erupting approximately 6 million years ago in the western portion of the field. Since that time the locus of activity progressed eastward producing over 600 volcanoes, culminating with the eruption of Sunset Crater Volcano approximately 900 years ago. Wupatki NM is located near the northeastern boundary of this field.

Visitors to the monument frequently inquire as to the location of the volcanoes that produced these flows and how long ago they were active. In an effort to answer these questions, a project was initiated with a Geologist-in Park intern in 2005 to characterize basalt from each of the mesa caps. Samples from flows that lie predominantly within the monument were analyzed petrographically and geochemically. Preliminary analyses suggest there are at least 8 distinct flows with unique geochemical signatures.

Petrographic analysis reveals that all of the flows are composed predominantly of olivine, augite, plagioclase and an opaque mineral. The individual flows vary in modal abundance of these minerals in the groundmass, the number and size of phenocrysts, and phenocryst assemblages. In the western portion of the monument, the many mesas are composed of only three individual flows. The first, represented on the mesa caps near the Lomaki and Citadel pueblos, is trachybasalt. The remaining two, located along the southern and western boundaries, are alkali olivine basalt. Five flows are exposed in the eastern portion of the monument. The Woodhouse Mesa flow near the Wupatiki ruin is compositionally basinite whereas those at Doney Mountain, near Wukoki Pueblo and within the Little Colorado Valley are alkaline olivine basalt. Correlation of these flows to known volcanic centers in the SFVF reveals that only a few remain whereas most have been covered by subsequent flows or eroded away. A proposal is currently being prepared to date those flows not correlated with known vents.