Fluvial strath terrace formation and soil development with tectonic implications along Cache Creek, California

The tectonic and climatic history of the Cache Creek watershed has been recorded by fluvial strath terraces from the late Pleistocene to the present. A total of nine terrace elevations at Wilson Valley and nine terrace elevations at Buck Island were mapped in detail. In lieu of numerical age control, the study of soil properties and soil development on these fluvial terraces has demonstrated that soils develop systematically with elevation and age. From this data a soil chronosequence has been established in Wilson Valley and at Buck Island. Using derived equations from soil properties and corresponding numerical ages (Harden and Taylor, 1983), age estimates have been calculated for each chronosequence. The estimated terrace ages for both study sites, based on the Ventura chronosequence data of Harden and Taylor range from 9 ka to 38 ka. The calculated uplift rates from these soil ages range from 0.26 to 0.36 mm/yr for the lower terraces and 1.25 to 2.0 mm/yr for the higher terraces. These uplift rate estimates suggest that the tectonic uplift/ incision rate in the upper Cache Creek drainage has not been constant during the late Pleistocene and have since decreased. The average estimated uplift rate calculated in this study of 1.1 mm/yr is thought to be higher than the estimates for the surrounding region. Terrace ages based on a constant average uplift rate of 1.1 mm/yr in Wilson Valley range from 3 ka for Qt2 and 67 ka for Qt9. Terraces ages at Buck Island based on the same rate range from 4 ka for Qt2 to 64 ka for Qt9. The difference in uplift rates between the upper Cache Creek drainage and surrounding region can be reasonably explained by a positive flower structure developing within a contractional duplex. Although this is a simple tectonic model trying to explain a complex system of strike-slip faults, it does agree with field observations, known fault geometry and calculations used in this study.