OSL AND IRSL DATING OF ALLUVIUM BURIED GREATER THAN 15 M IN THE EASTERN PLATTE RIVER VALLEY

As part of an ongoing study, we used optically stimulated luminescence dating (aka OSL) to determine how the eastern Platte River valley evolved near the towns of Ashland and Columbus Nebraska. The Platte River Valley alluvial fill ranges from about 14-18 m in thickness near Ashland to 30 to over 50 m thick near Columbus. Our goal for this portion of the project was to determine the nature of the fill in these different portions of the valley, and in particular to compare the ages of the fills. OSL provides a means of producing age estimates for the sandy fill in these portions of the valley, which is largely devoid of material suitable for ¹⁴C dating. Due to the relatively high environmental dose rates, OSL dating can only provide age estimates for Platte River alluvium deposited within the past ~100-150 ka. Based on our preliminary results, we can date sediments from the upper 10-15m of alluvium in the Columbus area and the entire alluvial fill within the Ashland area. We will apply Inferred Stimulated Luminescence (IRSL) to sediments buried below ~ 15 m in the Columbus area in an attempt to date these older sediments. The IRSL technique has been used to produce older ages than the OSL method, and in this application IRSL could prove useful in dating samples which would be otherwise undateable with existing methods.