THERMOLUMINESCENCE AND NEW C-14 AGE ESTIMATES FOR LATE QUATERNARY LOESSES IN SOUTHWESTERN NEBRASKA

Loess of late Quaternary age mantles most of Nebraska south of the Pla tte River Valley. At least five late Quaternary loesses are recognized : from oldest to youngest, one or more undifferentiated pre-Illinoian loesses, the Loveland Loess, the Gilman Canyon Loess, which exhibits a well developed soil and rests unconformably on the Sangamon soil, the Peoria Loess capped by the Brady soil, and the Bignell Loess, which i s distributed discontinuously. Previous research shows that the Lovela nd Loess is Illinoian, the Gilman Canyon Loess and Peoria Loess are Wi sconsin, and the Bignell Loess is Holocene. We present here the first thermoluminescence (TL) age estimates and new C-14 ages for these late Quaternary loesses at two key sections in southwestern Nebraska, the Eustis ash pit and the Bignell Hill road cut. TL age estimates from al l samples collected from Eustis ash pit and Bignell Hill were internal ly consistent. TL and C-14 age estimates from these two sections gener ally agree and support previous age determinations. The TL age estimat e on Loveland Loess indicates deposition at 163 ka. TL and radiocarbon age estimates indicate that Gilman Canyon Loess, believed to be depos ited during the Farmdale interstade, first began to accumulate at abou t 40 ka: the lower part of the Gilman Canyon Loess is 36 ka at Eustis and the middle of the unit is 30 ka at Bignell Hill. The lower and upp er parts of the Peoria Loess give age estimates of 24 ka and 17 ka, re spectively. TL age estimates for deposition of the Bignell Loess are 9 ka near the base, in agreement with radiocarbon age estimates, and 6 ka immediately below the modern soil, substantiating its Holocene age. Comparisons of TL age estimates with delta(18)O and insolation curves which show loess deposition during interglacial and interstadial as w ell as glacial periods, indicate that loess deposition on the Great Pl ains can occur under a variety of climatic conditions.