THE PETROLOGY OF PEAT LAYERS AT THE BURRO FLATS PALEOSEISMIC SITE ON THE SAN ANDREAS FAULT NEAR BANNING, CA: IMPLICATIONS FOR ESTABLISHING A RELIABLE, HIGH-PRECISION PALEOEARTHQUAKE CHRONOLOGY

Establishing robust paleoearthquake chronologies for paleoseismic sites along the southern San Andreas fault hinge upon acquiring high-precision radiocarbon age data from layers that bracket earthquake related deformation. Reliable, high-precision age data are especially critical for the San Andreas fault in southern California, where recurrence intervals as short as 45 years are known to occur (e.g., 1857 Fort Tejon and 1812 Wrightwood events). Out-of-sequence radiocarbon ages (older ages upsection) of several peat layers at the Burro Flats paleoseismic site near Banning, CA bring into question the reliability of some of the age data. For example, the age obtained from one peat layer is 120 +/- 50 radiocarbon years older than the underlying peat, a geologic improbability given that most other layers at the site yield in-sequence radiocarbon ages (younger ages upsection). Out-of-sequence ages in peat layers may result from contamination by old organic material. We have begun to inspect peat samples to identify, hand-separate, and date individual components of each layer. Peat samples contain varying amounts of ‘in situ’ plant fibers, both oriented perpendicular and parallel to bedding, and detrital components such as charcoal, seeds, and wood fragments. We interpret the layer-perpendicular fibers to represent roots and the layer-parallel fibers to represent annual reeds/grasses that, in winter, fall and orient themselves parallel to the ground surface. Radiocarbon ages of bulk peat samples therefore will yield a mixing age of old and young organic components, resulting in specious ages. Importantly, bulk ages may not represent the depositional age, critical for establishing a robust event chronology at the site. Results from six peat layers show that the detrital components consistently yield radiocarbon ages that exceed the layer-parallel fiber ages, in some cases by as much as 250 radiocarbon years. Furthermore the layer-parallel fibers yield in-sequence radiocarbon ages in cases where bulk samples show out-of-sequence radiocarbon ages. Hand separated layer-parallel fibers appear to record the true depositional age of the peat and will provide the most reliable radiocarbon ages to construct the chronology of paleoearthquakes on the San Andreas fault at this site.