PALEOMAGNETIC AND RADIOCARBON RECORD OF THE SEARLES LAKE FORMATION AT POISON CANYON, SAN BERNARDINO, CA

The Pleistocene-Holocene Searles Lake Formation records the last 150,000 years of pluvial Searles Lake in Searles Valley, California. The type locality for the Searles Lake Formation is a vertical section of 20 lithologic units in Poison Canyon. In spite of the extraordinary efforts by G.I. Smith, only the minimum age (>29 ka; conventional radiocarbon) of the older subunits (A, ab1 and ab2) is reliable. In this study, we collected fossils for radiocarbon accelerator mass spectrometer (AMS) analysis and rock samples for paleomagnetic analysis to ascertain the age of the older subunits. The AMS technique has a greater age range than conventional ¹⁴C and the older subunits should span two paleomagnetic excursions: Mono Lake Excursion (28-32 ka; MLE) and Laschamp Excursion (40.7 ka; LE) that should also allow correlation to similar deposits three kilometers west at the Tire Farm (informal name). At Poison Canyon, we obtained AMS radiocarbon ages of 42,900±100 ¹⁴C yrs B.P. (all ages uncalibrated radiocarbon years) for ab1, 31,830±170, 35,540±420 and 36,850±500 for ab3, 28,280±110 and 35,530±190 for ab5 and 31,410±160 for b. The Tire Farm fossils yielded ages of 40,210±400 (top of A) and 28,180±180 and 29,150±200 (base of A). The overlying subunit ab3 yielded ages of 30,840±190 and 47,150±190. The radiocarbon ages are not consistently in stratigraphic order; however, these differences may be attributable to interferences from hard water and the effect of changing paleomagnetic field intensity on radiocarbon production rate. Sediments thermally demagnetized to 600 C recorded reverse paleomagnetic polarity at both locations. The combined mean paleomagnetic directions are I=-37.5, D=180.2, alpha-95=19.5 (n=12) with a mean Virtual Geomagnetic Pole of 73.6 S, 231.8 E, alpha-95=20.6 (n=12). At Poison Canyon, the reverse polarity is in subunit ab2 whereas at the Tire Farm the reverse polarity is in subunit A. The reverse polarity and underlying radiocarbon ages, ~42 ka and 40 ka at Poison Canyon and Tire Farm, respectively, are consistent with the LE. The alternating mud and sand deposition of the A and ab subunits record fluctuating lake levels. Assuming the correlation to the LE is correct, then the Poison Canyon and Tire Farm areas straddled the paleo-shoreline between ~28 and ~40 ka.