OPTICALLY STIMULATED LUMINESCENCE DATING OF A HOLOCENE UNCONFORMITY WITHIN DELAWARE RIVER VALLEY ALLUVIAL DEPOSITS

Understanding the effects of past climate change on dynamic fluvial landforms is critical for predicting river responses to anthropogenic climate change. In the northern Delaware River valley, the middle terrace represents most of the Holocene sedimentary record, and contains an unconformity whose timing and cause are unknown. To determine whether this surface is related to known Holocene climatic events, we seek to constrain its timing using optically stimulated luminescence (OSL) dating.

Since terraces are discontinuous, correlation of the unconformity at different locations has not been proven. Ground-penetrating radar profiling at one location ~1 km² shows the approximate depth (~350 cm) of the unconformity, allowing targeted coring and sampling. From a 730 cm Geoprobe core, OSL samples were recovered at 277 cm, 332 cm, 630 cm, and 650 cm to bracket the unconformity.

Eolian dunes atop the upper terrace have been attributed to strong winds during the immediate postglacial. However, re-working during later arid periods is possible. If eolian activity occurred at the same time as the unconformity, dunes would provide critical information about climatic conditions. From a 240 cm Geoprobe core, OSL samples were recovered at 65 cm and 177 cm to determine timing of deposition.

OSL measurements were made on sand-sized (175-212 μm and 125-175 μm) quartz grains using a modified single-aliquot regenerative-dose (SAR) protocol with post-IR blue light stimulation. Equivalent dose data reliability was evaluated based on tests of dose recovery, preheat plateau, recycling ratios, and recuperation. Dose-rate data was derived from ICP-MS measurements of U, Th, K, and Rb, field moisture, and position details to assess contribution from cosmic rays.

Three of six samples demonstrate acceptable luminescence characteristics and provide ages between ~8.5-10 ka BP +/- 0.65 ka for deposits above the unconformity, and ~13 ka BP +/- 0.65 ka for the upper meter of the dune.

Three remaining samples give inconsistent dose recovery results with 160° C cutheat temperatures and varied preheat conditions. Alternative cutheat temperatures and linearly-modulated OSL characteristics are being investigated. Final results will place an upper age limit on the unconformity, and show whether the entire dune was deposited simultaneously.