ESTIMATING THE AGE OF TERMINAL FLUVIAL DEPOSITION on QUATERNARY FANS OF THE LOST RIVER BASIN, NORTHERN ROCKY MOUNTAINS, USA

Multiple cycles of late Pleistocene-Holocene alluvial fan aggradation and incision are preserved in the Lost River basin, Idaho. Poorly constrained timing of these depositional periods creates uncertainty in correlations with climate proxies. Estimates of terminal alluvial deposition and abandonment are provided by ²³⁰Th/U analysis of dense, inner laminae of pedogenic carbonate coatings that develop rapidly in the limestone-derived alluvium. These ages of surface abandonment complement depositional ages from ongoing OSL dating of the same fans.

Preliminary results from late Pleistocene surfaces of the Ramshorn fan include four U-Th analyses of pebble-coats with [U] = 0.5 -2.0 ppm and ²³⁰Th/²³²Th activity ratios of 5 to 25, showing they are moderately to highly suitable for precise, single-sample U-series dating. Analyses of two coatings from the two horizons of maximum CaCO³ accumulation in the RH2 fan-soil were sampled by drilling multiple laminae closest to their host pebble from cut and polished slabs. Results yield concordant ages of 24.7±1.8 and 25.2±2.4 ka, likely indicating a reliable minimum age for terminal deposition on this fan-surface. A clast deeper in the profile developed a thin, inconsistent coating, which yields a significantly younger age of 14.8±1.6 ka. The fourth sample yielded scattered, anomalously old U-series ages likely due to U loss, confirming that multiple samples are required for reliable fan ages and visual criteria alone are insufficient to identify carbonate that has maintained closed U-Th systems.

OSL results provide a maximum age for the same surface (RH2) at 14.96 ± 1.65 ka and another surface (RH1) on the same fan with an age of 39.44 ± 4.08. This suggests a period of active deposition across the fan surface during that time, which contrasts with very limited deposition during the Holocene. Additional U-series results will be compared with OSL ages to estimate timing of terminal fluvial deposition on surfaces of two other fans.