DETERMINING LAHAR STRATIGRAPHY USING RELATIVE DATING TECHNIQUES: EXAMPLES FROM THE SOUTH FLANK OF MOUNT SHASTA, CA

Pleistocene lahars on Mount Shasta in northern California provide various challenges to age determination. Radiocarbon dates may be obtained, but the origin of the carbon is suspect because the Shasta region is highly prone to forest fires. By using relative age dating techniques, including soil stratigraphy and weathering rind development, approximate age ranges can be assigned to the lahars. Glacial deposits may be used to calibrate these age ranges; unlike lahars, their origins are dependent on periods of cooler climate. On the south side of Mount Shasta, three age ranges have been used to delineate Pleistocene lahars: Panther Creek (~10,000 – 50,000 years), Bear Spring (~50,000 – 100,000 years), and Big Canyon Creek (200,000+ years). These categories are based on soil and rind development. Soil properties such as rubification, melanization, structure, consistency, and texture provide measurements of weathering over time. These properties have been quantified in a Profile Development Index (PDI) (Harden and Taylor, 1983). Weathering rind development is not as discerning, but still provides a way to separate ages. The lack of refinement is due to the high variation in rind thickness within a deposit. Finer grained andesites have smaller but better developed rinds than coarser grained andesites. Because rind development can be inhibited by silt coatings and clay skins, deposits with high contents of silt and clay may have thinner rinds.

Complications arise from the multiple layers of reworked ash that often cover Pleistocene deposits. Reworked ash caps often inhibit weathering of the underlying deposit. PDIs are calculated with and without ash layers to demonstrate the effects these ash layers have on weathering. Ash contains higher amounts of glass, which weathers faster than lithic fragments. As a result, the PDIs of the youngest Pleistocene deposits are raised when the ash cap is taken into account. Quantification of the effects of ash caps on weathering of underlying deposits may be possible by obtaining reliable carbon dates.