RADIOCARBON DATES OF THE UPPER PORTION OF THE PLEISTOCENE FOSSIL LAKE AREA, SOUTH-CENTRAL OREGON

Based upon the tephrochronology of the classical Fossil Lake area in Lake County, Oregon (Martin et al., 2005), the lithostratigraphic section extends from the late Irvingtonian through the Rancholabrean North American Land Mammal Ages, with the latest Irvingtonian and early Rancholabrean (~600 to 150 ka) represented by an unconformity. The youngest tephra correlates to the Trego Hot Springs tephra, dated at 23,200 +/- 300 ka. Therefore, the upper portion of the section is young enough for radiocarbon analyses. Two dates were previously reported: 10,140 +/- 130 rcbp (No. GX-31201-AMS) from salmonid remains secured from the white claystone near the top of the section (Unit 17 of Martin et al., 2005) and 6050 +/- 50 rcbp (No. GX-31202-AMS) from a lagomorph found in a burrow intruded into Unit 17 (Martin, 2006). Both dates were performed by Geochron Laboratories. Another Geochron Laboratories date from Unit 16 is 13,390 +/- 130 rcbp (No. GX-31198-AMS) based upon salmonid elements. Additional dates and refinements were accomplished through AMS radiocarbon dating through the PaleoResearch Institute. Unit 17 was redated at 12,850 +/- 50 rcbp (No. 09-82-092) based on mammal bone; Unit 18, the uppermost lithological unit at Fossil Lake was dated at 11,745 +/- 30 rcbp (No. 09-163-0550). Also, mammalian fossils were dated (15,480 +/- 70 rcbp, No. 09-163-0864) from just above the Trego Hot Springs tephra and just below (26,560 +/- 160 rcbp, No. 09-163-0536), both from Unit 14, indicating the accuracy of the AMS radiocarbon dates at Fossil Lake. Refined radiocarbon dating allows greater accuracy in the temporal assignment of the vertebrate assemblages and in the timing of events that occurred during deposition of the upper portion of the Fossil Lake Formation exposed in the Fossil Lake area.