OSL GEOCHRONOLOGY OF A LATE PLEISTOCENE SAND DUNE IN THE LOWER OHIO RIVER VALLEY

Four optically stimulated luminescence (OSL) dates were acquired on sands that compose morphologically distinct features interpreted as loess-capped transverse dunes, in order to tie eolian activity to paleoclimatic conditions. The sands are exposed in a quarry near Reo, Ind., where the dunes abut the western side of the Rockport Island Hills within the Ohio River Valley. The sediments composing the dune correspond to the Dune facies of the Atherton Formation in Indiana, or the Parkland sand in Illinois.

Our previous research identified six units in the quarry, but for this study only units 2 and 4, were dated. Unit 2 overlies unit 0 (lacustrine silt) and unit 1 (loess), and consists of crossbedded very fine sand with interbedded coarse silt (dune). Unit 4 is another crossbedded very fine sand with interbedded coarse silt (dune) that is overlain by a surface cap of loess (unit 5).

OSL samples were taken from the base and top of units 2 and 4, thereby bracketing the thinly bedded sandy silt of unit 3 that represents an interdunal deposit. The ages were as follows: unit 2 base, 16.37 ±1.28 ka (UNL1639), unit 2 top, 17.54 ±1.68 ka (UNL1640), unit 4 base, 16.36 ±1.08 ka (UNL1641), unit 4 top, 17.45 ±1.38 ka (UNL1642). The slightly incongruous 16 ka age from the base of unit 2 is likely because of uncertainty in the water saturation history (increased water content reduces the dosing rate for a sample). The basal unit 2 sample was calculated using a water content of 25 percent while the upper sample of unit 2 was measured at 3.0 percent. Basal unit 4 was measured at 7.1 percent, with the upper unit 4 sample measuring 3.3 percent. A wetland behind the dune was drained in the mid 1870's, which likely caused the water saturation history to be underestimated. All four OSL ages fall within the 1ó error.

The OSL dates contradict an earlier radiocarbon date that was older than 46,000 yr BP (Beta-217287) from the lower 20 cm of unit 2. The dates indicate that dune formation was under way shortly after the Wisconsinan glacial maximum. The dunes indicate that wind velocities and directions, moisture conditions, and sediment supplies were sufficient at that time to generate these topographically impressive eolian features. The dates also confirm that the radiocarbon dated organics were reworked mid- or pre-Wisconsinan organics.