Soils Formed From Various till Deposits in the Lake Tahoe Basin and Their Use in Understanding Neotectonics.

Ongoing work in the Lake Tahoe Basin by the USGS and others continues to support the notion that major tectonic activity (Richter 7 or greater) has occurred within the last 20 thousand years (ka), causing surface offset of up to 70 meters in Tahoe aged moraines (~62ka) and 40 meter in Tioga (~21ka). Soils were sampled and analyzed along the crests of the right-lateral moraines in Meeks Creek of both Tahoe and Tioga tills to see if variations in soil development could be used to differentiate landscape ages and help better understand the neotectonic setting of the Basin. Work done elsewhere along the Sierra Nevada’s has shown variations in soils of Tahoe and Tioga ages, but no extensive work has been done along the west shore of Lake Tahoe. Field characterizations of Tahoe and Tioga soils show few differentiating characteristics, although some Tahoe soils are slightly redder (some 10YR hues instead of the 2.5Y hues in Tioga soils) and sometimes have finer textures (sandy loams instead of loamy sands). Lab analyzes included particle size distribution, selective dissolution, x-ray diffraction, optical mineralogy, and differential scanning calorimetry–thermogravimetry (DSC-TG). Results show that Tahoe aged soils have more gibbsite (10-50% by mass in the clay fraction compared to 0-10% in Tioga soils), more dithionite extractable iron (0.2-0.5% by mass in the whole soil compared to 0.1-0.2% in Tioga soils), and a greater degree of physical grain weathering in the very fine sand fraction (50-100μm). From these data we see it is the combination of minor differences in micro-morphology and mineralogy that can be used to differentiate soils formed from different aged till deposits. These findings will help to more extensively map the soils and landscape in this moraine setting and hopefully dissect and understand the recent tectonic activity in the Tahoe Basin.