DOES OSL DATING WORK ON GLACIALLY DERIVED DEPOSITS IN ALPINE AND CONTINENTAL SETTINGS? SOME ANSWERS TO THIS QUESTION WITH EXAMPLES FROM FIVE CASE SITES

Although optically stimulated luminescence (OSL) of quartz and potassium feldspar grains is not ideally suited to dating deposits directly related to ice sheet movement, glaciofluvial and glaciolacustrine reworking and subsequent re-deposition shortly thereafter can provide deposits more amenable to OSL dating. Accurately identifying, finding, and sampling these deposits and landforms is key to assessing whether OSL single grain and single aliquot dating produces reliable and precise ages. When we have a better knowledge of which glacially derived deposits the OSL methods work optimally in, it can be more profitably administered to sediments from older events as well as providing useful parameters for climatic models.

Regional source geology is also as important as the depositional environment and this fact is usually little appreciated. For example, sedimentary quartz grains that have endured several deposition and erosion cycles usually present better luminescence data than magmatic or volcanic quartz grains that have been transported short distances or were only deposited once. When quartz produces low signals, feldspar is often used to good effect as incomplete bleaching of feldspar is less of a problem than once believed.

Several case studies from sample collections in Michigan, New England, Washington, Oregon, and the Colorado Rockies will be presented. Samples were collected from a variety of proximal and distal ice-sheet deposits. The equivalent dose distributions found in both continental and alpine deposits that range from fifteen to one hundred thousand years old show that wide scatter and poor resolution can be improved with larger data sets and when sampling locations are well identified and correlated. Results related to luminescence characteristics, equivalent dose distributions, field locations, and comparisons to previous age control will be discussed.