OPTICALLY STIMULATED LUMINESCENCE DATING OF LOESS AND SAND SHEETS IN THE ANTON SCARP TRENCH, NORTHEAST COLORADO

In 2003, Vince Matthews (Colorado Geological Survey) identified a 95-mile-long lineament in northeastern Colorado. In 2004, a trench was excavated into the scarp near the town of Anton to investigate the age and origin of the scarp and to explore the possibility that it is the product of Quaternary faulting. From the top down, the trench revealed (1) loess, 4.5 m thick, (2) eolian? sheet sand, 4.2 m thick, (3) loess, 7.0 m thick, and (4) eolian dune sand, 4.5 m thick, the upper part of which contains a calcic soil. From about the midpoint to the bottom of the scarp, alluvium-filled channels (1.0-4.7 m deep) crosscut the scarp and the units in the lower part of the section.

Eleven samples were collected from the trench for luminescence dating and twelve samples were collected for 14C dating. Luminescence samples were collected from the two loess units and from the eolian? sheet sand between, near the base of the units and where possible at the top. Two luminescence samples in upper loess (identified as Peoria Loess) were dated with IRSL and Blue-light OSL. The ages obtained are 15.6 ± .85 ka, 14.0 ± .68 ka and 13.8 ± .63 ka. These ages coincide well with those reported in other recent studies on the Peoria Loess that gave 13.7 ± 0.6 ka and 14.5 ± 0.7 ka for the upper loess and 16.6 ± 0.9 ka and 18.4 ± 0.9 ka for the base in Nebraska. The lower loess, identified when sampled as being of Bull Lake age (about 186-127 ka), was also dated using IRSL and Blue-light OSL. The luminescence ages were older than the upper loess, but not excessively so, at 26.0 ± 1.8 ka and 31.0 ± 3.1 ka. This indicates the lower loess unit is likely either Peoria Loess or Gilman Canyon Loess, which has been reported with TL ages ranging from 30-22 ka. The intervening eolian sheet sand has not yet been dated using luminescence. No radiocarbon datable materials were observed in either the upper or lower loess or in the sheet sand.