FANGLOMERATE DATING IN THE COACHELLA VALLEY, SOUTHERN CALIFORNIA: MULTI GRAIN AND SINGLE GRAIN OSL RESULTS

Alluvial fans and their drainages in arid and semi-arid regions like southern California, record paleoenvironmental changes and seismic activity. In order to interpret this valuable information an appropriate chronological method is required. OSL dating can be used in these settings for events occurring over Late Quaternary timescales. However, current OSL approaches are only suited to the fine sand deposits which are rarely present as layers or lenses in the fan architecture; therefore, we tried to expand the applicability of OSL dating by sampling the coarse fraction fanglomerates and extracting the fine sand matrix for dating. Regarding this new technique, partial bleaching --because the possibility of poor sunlight bleaching during sediment transport is high-- and dose-rate heterogeneity --fanglomerates of very fine sand and silt <90 µm up to clasts as big as ~5 cm in diameter-- should be considered.

We have two fanglomerate sampling locations in the Coachella Valley and to test the accuracy of the fanglomerate dating approach, we collected samples in stratigraphic superposition; collected from fine sand lenses where available; and one of our fanglomerate sampling sites correlates with existing terrestrial cosmogenic nuclide (TCN) chronology.

In the OSL lab, the fine sand matrix was separated from coarser clasts and then standard mineral preparation methods were employed. We conducted both multi-grain aliquot and single-grain measurement on quartz and K-rich feldspar. Quartz analysis indicated dim OSL signal with poor OSL characteristics. On the other hand, feldspar showed bright and well-behaved infrared stimulated luminescence (IRSL) signals. We employed the post-IR-IRSL 225°C (pIRIR₂₂₅) method and measured lab residual by bleaching aliquots for 1 hr in sunlight. Moreover, feldspar anomalous fading test was performed on multi-grain aliquots.

Considering dose-rate heterogeneity, high resolution gamma spectrometry was utilized for >2 mm and <2 mm proportions of the fanglomerate samples; in addition, 3 portions of each sample (>2 mm, <2 mm, and a mixture of the two sizes) was analyzed using ICP/ OES-MS.

We will present results of our investigations into partial bleaching and dose-rate heterogeneity for fanglomerate samples, and compare ages to OSL results from fine sand deposits and TCN chronology.