SOIL CHRONOSEQUENCES AND RETROGRADE PEDOGENESIS ON LAVA FLOWS, SAN FRANCISCO VOLCANIC FIELD

The San Francisco Volcanic Field is located on the southern edge of the Colorado Plateau and includes more than 600 basaltic volcanoes of Tertiary and Quaternary age (Tanaka et al., 1986). Many of the volcanoes and their respective lava flows have been studied thoroughly and confidently dated. Two lava flows within the volcanic field that have been previously studied are The Sproul and North Mesa. The Sproul is believed to be 20 ka and the North Mesa is 850 ka. The development of a soil chronosequence will allow us to estimate ages on undated flows and rates of volcanic landscape evolution. A chronosequence was developed utilizing calcium carbonate equivalency, Harden Soil Development Indices, and particle size analysis. The calcium carbonate equivalency measured on soils from both The Sproul and North Mesa identified CaCO3 buildup with increasing age. In addition, soil development indices increase with age on lava flows until the soil reaches a certain stage of development, and then they decrease. Similarly, particle size analysis indicated that silt and clay percentage also increases with age on lava flows until the soil reaches a certain age. Finally, based on paleomagnetic dating and similar characteristic remnant magnetism, The Sproul (declination = 9.5°, inclination = 26.9°, α95 = 6.4. K = 25.3) is about the same age as Merriam Crater (declination = 9°, inclination = 29°, α95 = 9.1, K = 186). The soil chronosequence developed from the soils on lava flows within the San Fransisco Volcanic Field indicates that at some time between about 70 ka and 850 ka, soils on lava flows begin to degrade, sediments are eroded, surface horizons thin, and Bkk horizons are closer to the surface, leading to retrograde pedogenesis.