DATING SEISMIC-RELATED FANGLOMERATES IN THE COACHELLA VALLEY, SOUTHERN CALIFORNIA

In tectonically active arid and semi-arid regions like southern California, alluvial fans and their drainages record not only paleoenvironmental change but also fault movement. To be able to decipher this precious information a suitable chronological method is required. For events occurring over Late Quaternary timescales, OSL dating is a chronological method that can be used in these settings; however, current OSL approaches are only suited to the fine sand deposits which are present as layers or lenses in the fan architecture. The problem with this sampling approach is that in many instances such deposits are rare; therefore, in this study we are trying to establish a luminescence method for dating fanglomerates by sampling the coarse fraction and extracting the fine sand matrix for dating. Regarding this new technique, some issues such as partial bleaching and dose rate heterogeneity should be considered; yet this is worthwhile since this method can greatly expand the applicability of OSL dating in similar settings.

We have five main sampling locations in the Coachella Valley; at two of these we collected fanglomerate samples in black plastic bags at night. Later, in the OSL lab, the fine sand matrix was separated from coarse clasts and then standard mineral preparation methods were employed. In previous studies in southern California quartz luminescence has been shown to work well but in others only feldspar methods were successful. Therefore, we have decided to investigate both quartz and feldspar. We will also perform single grain analysis for both quartz and feldspar grains because the possibility of poor sunlight bleaching during sediment transport is high. Considering dose rate calculations, the heterogeneity in the environment of fanglomerates (composed of fine sand smaller than 90µm up to clasts as big as ~5cm in diameter) will be tackled using high resolution gamma spectrometry analyses and modelling dose rate by DosiVox software package.

Furthermore, we have devised a sampling methodology to enable us to test the accuracy of the fanglomerate dating approach: we collected samples in stratigraphic superposition; targeted fine sand lenses; and one of our fanglomerate sampling sites correlates with existing TCN chronology.

Preliminary results of quartz and feldspar single aliquot analysis will be presented.