3 HOLOCENE TEPHRAS IDENTIFIED IN LACUSTRINE SEDIMENT CORES FROM THE WONDER LAKE AREA, DENALI-NATIONAL-PARK AND PRESERVE, ALASKA, USA

Eighteen percussion piston cores were recovered from Wonder Lake and t hree nearby kettle ponds in Denali National Park and Preserve, Alaska. Three prominent tephra deposits, two felsic and one mafic, have been recognized in the upper 1 to 3 m of the cores. Because of the relative ly low magnetic susceptibility (MS) of the lacustrine sediment, tephra s appear as prominent MS peaks allowing confident correlation of core stratigraphies. These MS-based correlations are supported by microprob e geochemical analyses of 11 tephra samples from six of the cores. Fiv e accelerator mass spectrometer (AMS) radiocarbon ages, determined fro m terrestrial plant remains, closely constrain the timing of these ash fall events. Microprobe results indicate that the youngest tephra is c orrelative with the Jarvis Ash, which has an accepted age of 3660 +/- 125 C-14 yr BP. The next older felsic tephra is correlative with the m iddle Holocene Oshetna tephra. New C-14 ages reported in this study su ggest that this regionally extensive tephra was deposited about 6000 C -14 Yr BP. Neither of these tephras has previously been identified thi s far west, indicating that both are significantly more extensive than previously thought. A fine-grained basaltic ashfall deposit, identifi ed in several of the Wonder Lake cores and dated to ca. 10,000 C-14 yr BP, is a newly discovered tephra that provides an important stratigra phic marker horizon at the Pleistocene-Holocene boundary in central Al aska.