DATING THE GREENWOOD MORAINE (INDIANA) BY OPTICALLY STIMULATED LUMINESCENCE DATING (OSL): AN ALL TOO COMMON STORY OF APPLYING A TECHNIQUE IN A DIFFICULT ENVIRONMENT IN THE HOPE OF SHINING LIGHT ON AN EXCITING IDEA

The Laurentide Ice Sheet deglaciation in south-central Indiana is marked by a series of moraines. Of interest a field survey in 2014 identified and mapped an outwash deposit, grading directly to the Greenwood Moraine, a major moraine inboard of the last glacial maximum position.

In an attempt to constrain the timing of this geomorphic feature, samples for optically stimulated luminescence dating (OSL) were retrieved from the outwash sedimentary unit. OSL dating is a technique able to measure the time elapsed since the last exposure to sunlight, hence the burial age of a sediment. Samples were collected at various distances away from the moraine, spread from 0.1 to 5 km, to test the effect of transport distance on luminescence characteristics and precision of ages. All samples were collected with a light-proof geoprobe core, between 1.5 and 4 m depth in order to avoid issues associated with pedogenesis. Most samples were dated with quartz minerals and a double comparison was made with K-feldspar for one sample. Difficulties were encountered during the OSL dating, most notably partial bleaching. If a mineral grain of sand (150 – 250 mm; quartz or feldspar) is not efficiently exposed to sunlight during its last sedimentary cycle, then that grain will carry a residual age. Ideally the residual age should be zero. Partial bleaching, if not properly taken into account, skew the calculated OSL age and leads to an overestimation. In addition, it was found that some of these sediments suffer from uranium disequilibrium (excess in radium 226). Despite all of this we obtained an age of approximately 21 ka, in line with the geological expectation.

This presentation will highlight the methodological procedures applied here to unravel the dating complexities of such a depositional environment.