OXYGEN ISOTOPE STAGE 4 SEDIMENT DOMINATES THE U.S. SOUTHWEST ALLUVIAL RECORD

Much of the U.S. southwestern mid- to late-Quaternary basin and fluvial fill records appear to be dominated by a regionally isochronous fill event that occurred during Oxygen Isotope Stage 4 (OIS4, 75 to 60 ka). This observation is based on soils stratigraphy, fill terrace or fan geomorphology, relative sediment volumes, and terrestrial cosmogenic nuclide (TCN) exposure dating in the Providence Mountains (Mojave), Queen Valley (eastern California shear zone) and Grand Canyon (Arizona). Minimum TCN exposure ages (boulders, pebble pavement, and subsurface samples, no compensation for erosion or mixing, but adjusted for inheritance where possible) on the most recent fills to dominate these respective areas are: PM-Qf3, 58 ± 22 ka (n=17), QV-Qf2, 69 ± 21 ka (n=6), and GC-S3/M3, 52 ± 16 ka (n=3), total uncertainties (precision and systematic) at 2σ. Despite measurable inheritances in all three areas, and the dominance of the fill by coarse cobble gravel which makes exposure dating difficult, the minimum ages of these fills are statistically distinguishable from the next older and younger fill units in each area. In the GC and PM other dating methods corroborate these findings. We recognize that in areas where glaciofluvial sediments dominate the sediment flux, such as the eastern flank of the Sierra Nevada, OIS2 sediment appears to be the dominant fill record.

If one or more widespread synchronous sedimentation events can be established in fan and terrace records: (i) undated records may be correlated regionally without the expense of analytical dating; (ii) slip rates and seismic recurrence intervals may be better established; and (iii) models of the nature basin and fluvial sedimentation should consider a strong climate influence during OIS4. An explanation for why in the U.S. southwest (and elsewhere) the OIS4 alluvial fills are significantly larger than OIS2 fills, in contrast to alpine glacier records in the same area, will provide important insight into sediment production, storage, and evacuation at timescales of glacial cycles.