Morphology and Mineralogy of Soils Derived from Pleistocene Till Deposits Along the Right Lateral Moraine Crest of Meeks Creek, West Shore Lake Tahoe, California.

摘要

Within the Meeks Creek drainage along the west shore of Lake Tahoe there is clear evidence of at least two major glacial advances, correlating to Tioga and Tahoe glacial episodes. Recent cosmogenic exposure dating of surface boulders along the crests of the right lateral moraine complex yielded minimal ages of 20.8±1.4 ka and 69.2±4.8 ka for the Tioga and Tahoe glaciations respectively.;Two (2) pedons from the Tioga moraine crest (Ti1 and Ti2) and three (3) pedons from the Tahoe (Th2, Th3, Th4) moraine crest were exposed, described and sampled for analysis in the laboratory. Differentiation of soils formed from Tioga and Tahoe till deposits in the field was extremely difficult. Overall soil colors are dominated with 2.5Y and 10YR hues, with high values and chromas. Tahoe soils do tend to be slightly redder than Tioga soils as a group. Soil textures range from sands to loamy sands, with a few sandy loam textures in the more developed Tahoe soils (Th2 and Th3). Rock fragment (RF) content (>2mm in diameter) ranges from 2-80% (by volume) across all soils, with a general increase in RF with depth. All soils are classified as sandy-skeletal, mixed, frigid Dystric Xerorthents, with the exception of Th3 which is a loamy-skeletal, mixed, superactive, frigid Typic Haploxerult.;Selective dissolution of the fine earth fractions (FEF) (<2mm in diameter) reveals greater quantities of dithionite-extractable iron (Fed) in Tahoe soils. Fed values in Tioga soils range from 0.5-1.8 g kg-1, with a depth weighted average (DWA) of 1.39 g kg-1 and 1.23 g kg-1 for Ti1 and Ti2 respectively. Fed values in Tahoe soils range from 1-5.1 g kg-1, with DWA's of 2.19 g kg-1, 3.83 g kg-1, and 2.06 g kg-1 for Th2, Th3, and Th4 respectively. Increased Fe¬d values in Tahoe soils reflects the greater quantity of secondary Fe-oxyhydroxides accumulated in these soils as mineral weathering processes have proceeded over a greater period of time.;X-ray diffraction (XRD) analysis was performed on the clay fraction (<2&mgr;m) (CF) using a Rigaku Ultima IV X-ray diffractometer with a Cu-tube. Treatments included: K-saturation (K-25), Mg-saturation (Mg-25), Mg-saturation/glycerol-solvation (Gly-25), and heating K-saturation slides for 2 hours at 350°C (K-350) and 550°C (K-550). Results showed that Tioga and Tahoe soils have similar mineral assemblages in the CF, but in varying quantities. Vermiculite dominates the clay mineralogy of Tioga soils, while gibbsite and kaolinite have the most intense XRD peaks in Tahoe soils. The lack of a clean collapse of the 14Å peak of vermiculite with K-25 in Tioga and Tahoe soils may indicate a lowering of the charge of the interlayer sheets of the vermiculite, which could lead to Al-hydroxy interlayering as vermiculite alteration continues.;Quantification of gibbsite in the CF was measured using a combined differential scanning calorimetry and thermogravimetry (DSC-TG) approach. Peroxide treated clay samples were combusted with a Netszch STA 409 PC Luxx furnace. Sample mass loss was compared to a standard curve in order to estimate total gibbsite quantities in the clay samples. The CF of all Tahoe soils have significantly greater gibbsite quantities than any Tioga soil. DWA quantities for Tioga soils are 1.2% and 8.9% for Ti1 and Ti2 respectively, while Tahoe soils contain 18.4%, 43.9%, and 32.0% for Th2, Th3, and Th4 respectively. The gibbsite and kaolinite distribution generally show an accumulation with depth, with the maximum quantity in many soils being at the lowest depth.;The very fine sand (VFS) fraction (0.053-0.10mm in diameter) of Tioga and Tahoe soils contain varying quantities of are biotite, hornblende, feldspar (potassium and plagioclase), quartz, and volcanic glass. Overall, grain frequencies and ratios did not significantly differ between Tioga and Tahoe soils. Semi-quantitative analysis of hornblende weathering, as assessed by observable alteration of the grains, did however differentiate Tioga and Tahoe soils. Hornblende grains were viewed under a petrographic microscope and placed into one of three categories based on their weathered appearance: low, medium, or high. Tahoe soils contain about twice the amount of medium weathered grains and two to five times more highly weathered grains than Tioga soils. Volcanic glass accounts for ∼5% of grains in the VFS fraction in the surface of all pedons and generally decreases with depth and is absent below ∼60cm. This distribution of glass suggests post Tioga deposition across the landscape and may be a key to further constraining the ages of the soils, if the origin and age of the glass can be determined.;By understanding the variations between and within Tioga and Tahoe soils, we can reach a better understanding of the soil forming processes occurring on this landscape. (Abstract shortened by UMI.).