EVIDENCE FOR A LONG-LIVED PLEISTOCENE LAKE, CARRIZO PLAIN, CALIFORNIA

Two recently obtained cores provide evidence of a long-lived lake that occupied the Carrizo Plain during the Pleistocene. Both cores come from an elevation of 584 masl on a portion of the former lake floor that was abandoned during the Holocene.

The longer of the two cores (~42 m) has been sampled for a variety of analytical studies (e.g., palynology, isotopic chemistry, environmental magnetism, and SEM-petrography). The magnetic susceptibility signal contains two notable features corresponding to lithologies consistent with reducing conditions. The higher of these features occurs near the surface, the lower at ~18 m depth. A 14C date on charcoal from the upper reduced zone places the top of this zone at no older than 17.74 ± 0.330 14C ka (20.24-22.00 cal ka). This date is consistent with OSL dates on geomorphic features associated with a highstand at 595 masl. The youngest age of the highstand shoreline was constrained by an OSL date of 16.7 ka from the top of the corresponding clay dune.

Assuming that reducing conditions correspond to deep water, the new 14C date suggests that the upper reduced zone represents a Stage 2 pluvial maximum lake in the Carrizo Plain. If the lower reduced zone has a similar origin, then the Carrizo Plain has held a lake since well before Stage 6 time. This implication substantially extends the time interval of lacustrine deposition on the floor of the Carrizo Plain and, therefore, the time since the basin lost external drainage.

The present lake floor is tilted due to deformation likely associated with the nearby San Andreas Fault. The lake sediment cores were taken from a surface above the present lake (582 masl) presumably abandoned by this tilting sometime after the maximum highstand. Thus, any record of Holocene deposition, if it ever existed at either core site, has been lost, probably by deflation.