COSMOGENIC EXPOSURE AGES FOR TIOGA AND TAHOE AGE MORAINES AT MEEKS BAY, LAKE TAHOE, CALIFORNIA

On the west side of Lake Tahoe near Meeks Bay, California, is a well-preserved morainal complex. The complex is constructed of granitic till from the late Pleistocene 'Tioga' and 'Tahoe' glacial advances. Cosmogenic nuclide surface exposure dates for these moraines are derived from cosmogenic nuclides that begin accumulating when the moraine crest boulders are exposed at the surface. In this study the cosmogenic nuclides are ²⁶Al and ¹⁰Be. The concentrations of ²⁶Al and ¹⁰Be increase over time at empirically derived production rates. Erosion of the moraine crest and exhumation of fresh unexposed or shielded boulders can cause the cosmogenic exposure age to be younger than the age of the moraine. To minimize the shielding effect caused by subsequent erosion of the moraine crest and exhumation of fresh (unexposed) boulders only the most prominent moraine crest boulders (2.3 to 4.0 m tall) were sampled. Chiseled flakes (2-5 cm thick) collected at the top of the boulders were processed and analyzed for ²⁶Al and ¹⁰Be. The exposure ages were corrected for variations of the magnetic field intensity and pole position using the Stone, (2000) latitude and altitude correction factors and a sea level high latitude ¹⁰Be production rate of 5.06. ²⁶Al ages were calculated using the Stone, (2000) latitude and altitude correction factors, assuming a ²⁶Al/¹⁰Be production ratio of 6. The mean (²⁶Al and ¹⁰Be) age for the 'Tioga' moraine at Meeks Bay is 20.4 ±0.7 ka yrs B.P.(n=4). The mean (²⁶Al and ¹⁰Be) age for the 'Tahoe' moraine at Meeks Bay is 62.0 ±1.3 ka yrs B.P.(n=2). These cosmogenic exposure ages must strictly be considered minimum limiting ages that underestimate the true depositional age of the moraine. This underestimation of the moraine age is attributed to two processes: (1) erosion of the moraine and exhumation of shielded boulders, and (2) the unknown attenuation of the cosmic neutron flux caused by snow cover on boulders for as much as five to six months of the year. These processes could cause an underestimation of the apparent age.