SEDIMENTOLOGICAL AND GEOCHRONOLOGICAL EVIDENCE FOR LARGE-MAGNITUDE PALEOFLOODS IN THE DESERT REGION OF BAJA CALFORNIA SUR, MEXICO

Sedimentological analyses and optical dating results from extensive alluvial fan terraces in the desert region of Baja California Sur, Mexico, suggest that a very different climatic regime existed prior to the Holocene (~11.7 ka). Recently, researchers have discovered a more stable infrared-stimulated luminescence (IRSL) signal within K-feldspar, termed the 'post-IR IRSL' (Buylaert et al., 2009). This study measures the post-IR IRSL signal within individual grains of K-feldspar. The use of single grain measurements in luminescence dating allows for confident identification and circumvention of incomplete IRSL signal zeroing, a problem characteristic of dating alluvial fan sediments. By sampling in high density (typically four sediment samples per stratigraphic section) over a wide spatial extent (15 sites spread over 145 kilometers) we are able to assess the regularity of depositional events between and within terrace units. We demonstrate a regionally-consistent pattern of more recent, lower magnitude terrace deposits overlying pre-Holocene, larger magnitude deposits. Antidune bedforms and transported boulders found within these pre-Holocene units suggest paleoflood discharges up to ~13,000 m³/s. We interpret this change in depositional style to be reflective of a regional shift in climatic regime. The modern recurrence of these floods has important implications for local communities as well as, potentially, the southwest United States.

References:

Buylaert, J.P., Murray, A.S., Thomsen, K.J., Jain, M., 2009. Testing the potential of an elevated temperature IRSL signal from K-feldspar. Radiation Measurements 44, 560-565.