EXPLORATORY QUARTZ AND FELDSPAR LUMINESCENCE GEOCHRONOLOGY OF PRE-WISCONSIN SEDIMENTS, SOUTH-CENTRAL INDIANA

Luminescence dating is already established as a geochronological tool to study Wisconsin Episode glaciated and ice marginal areas in Indiana. Application of the technique to older sediments however has been rare. During the past year, we have assessed the suitability of both quartz and feldspar coarse-grain luminescence dating as a tool for understanding landscape evolution, paleoenvironments and pedogenesis in both glaciated and ice-marginal areas in south central Indiana during pre-Wisconsin times. Samples analyzed come from both exposed sections and cored sequences. A diverse set of sampling protocols has been used, although we highlight here core samples, given that many of the pre-Wisconsin sediments in the region appear only in subsurface sequences. We have used the small aliquot approach for all of our measurements, exploring both quartz OSL as well as feldspar post-IR IRSL protocols.

The most common sediments correspond to glacifluvial Illinoian (MIS6) deposits, with ages ranging from 150 to 130 ka. A key factor in obtaining these ages is the relatively high level at which dose saturation appears for quartz in some subsurface samples in central Indiana. 2D0 values between 200 and 600 Gy help constrain age of MIS6 sediments given the relatively low (1-2 Gy/ka) dose rates in most sediment packages. Notably, those 2D0 values are higher than values observed on Wisconsin age sediments from the same region. A possible difference in provenance might explain the discrepancy. When tested against quartz ages, post-IR IRSL protocols in feldspar appear to recover burial doses well, and potentially could extend the reach of the technique towards ~250 ka.

In many cases, uniform Illinoian sediment packages up to 60 m thick appear on top of gravel lag directly over bedrock, suggesting either that deep flushing of ice marginal areas occurred between largest glacial episodes, or that indeed the Illinoian glaciation was between the most extensive glacial episodes in central Indiana.

We conclude that the technique can be applied successfully to sediments in this age bracket, and that further exploration of the possibility that luminescence properties might be useful to discriminate provenance of sediments is warranted.