MULTI-MILLENNIAL TIMESCALES OF DISTAL TEPHRA REWORKING IN THE WESTERN GREAT BASIN IDENTIFIED BY LUMINESCENCE DATING

Tephra deposition occurs within days to a few years after an eruption event and has been considered geologically instantaneous, particularly with respect to the precision of dating techniques commonly applied to Quaternary deposits. Sedimentary features such as changes in bed thickness and current structures, however, suggest that many tephra deposits do not represent original airfall and have been reworked after deposition. Where tephras are found over broad areas in a variety of depositional settings, it is reasonable to question the time range of deposition for the event.

In the Lahontan basin, tephras have been used extensively to document lake-levels at the times of eruptions, providing quantitative evidence of lake-level change through time. The assumed isochronicity of the Wono (~27 ka), Trego Hot Springs (~24 ka), and Mazama (~7 ka) tephras, however, has led to conflicting interpretations of lake level at the times that these tephras were deposited. In this work, we present results from field observations, particle size analysis, and luminescence geochronology applied to known age tephras across a range of depositional environments in the Lahontan basin to define the timescales over which reworking occurs.

Our results show that the post-IR₅₀IR₂₂₅ signal derived from tephra deposits in the Lahontan basin are reproducible and display no significant anomalous signal fading. Importantly, the post-IR₅₀IR₂₂₅ signals are also sensitive to light and, consequently, any post-depositional reworking event provided that light exposure was sufficient during transport. As such, any significant underestimation in a tephras ‘apparent’ post-IR₅₀IR₂₂₅ age relative to its known age may be suggestive of a post-depositional reworking event if all uncertainties can be accounted for. Even with conservative uncertainty estimates in our age calculations, our results suggest that tephra reworking can occur up to several thousands of years after their initial deposition. Importantly, our results also indicate that distinguishing between primary and secondary (reworked) tephras may not always be possible by field observations or grain size analyses alone.