A SOIL CHRONOSEQUENCE FROM LOESS DEPOSITS ON LATE PLEISTOCENE LAVA FLOWS, NORTHERN ARIZONA, USA: RESULTS FROM THE SAN FRANCISCO VOLCANIC FIELD REU

The San Francisco Volcanic Field in northern Arizona is home to ~600 cinder cones, many of which have been dated by a variety of methods. In this study, a soil chronosequence was developed to better constrain ages on Strawberry and SP Craters’ lava flows. These lava flows are capped by relatively thin (~1 m) eolian mantles in which dust accumulates to form soil over time. Methods employed included Harden Development Indices as well as particle size and soil nutrient analysis.

Soil profiles from the SP flow have more strongly developed columnar soil structure, higher silt and clay content, and higher calcium carbonate concentrations than those on the Strawberry flow, suggesting that SP soils are older than Strawberry soils. Field observations were supported by Harden Development Indices and particle size analysis. Both phosphate and nitrate appeared to be concentrated in upper, eolian-derived horizons (Av, Bw) and in carbonate horizons (Bk). Higher concentrations of phosphate and nitrate were found in SP soils than in Strawberry soils, suggesting that the nutrients may be useful as relative age indicators for these soils.

Preliminary OSL ages from the loess deposits indicate that Strawberry soils are ~12.3 ka whereas SP soils are ~ 32.7 ka, supporting the documented soil development indices. Cosmogenic He³ ages from the SP and Strawberry cones are both ~50 ka. OSL ages and soil stratigraphic interpretations of the Strawberry soil profiles identified a buried soil developed in eolian deposits directly atop the lava flow. This well-developed, buried soil was subsequently buried by another tephra layer, possibly from the nearby Merriam Crater that erupted about 20 ka, suggesting that our OSL age is for a Merriam-aged (or other “post-Strawberry”) soil and not a Strawberry-aged soil. The eruption of Sunset Crater in approximately AD 1100 deposited a tephra blanket across a wide area, providing the youngest and least developed soil for our chronosequence.

Results from this study yield clear trends for soil development with age that are supplemented by the OSL and ³He ages for SP and Strawberry. This study helps to demonstrate the efficacy of combining multiple dating methods to yield complimentary results, and contributes to our understanding of landscape evolution in this monogenetic volcanic field.