A RECORD OF LATE QUATERNARY ENVIRONMENTAL CHANGE: STRATIGRAPHY, PALEOECOLOGY, AND TEPHROCHRONOLOGY FROM MARIPOSA CREEK, YUKON TERRITORY, CANADA

Late Quaternary stratigraphy, tephra, pollen, plant macrofossils, and insect fossils were studied from a placer mine site along Mariposa Creek in west-central Yukon. Stratigraphy from the eastern placer pit face (Wall A) appears relatively unaffected by cryoturbation and erosional events suggesting a continuous record of environmental change dating back ~ 60 ka. Samples of possible early-Middle Wisconsinan-age (ca. 45 000 to 60 000 B.P.) from Organic Unit 1 indicate an open forest-shrub/tundra environment that was slightly colder and drier than present. Floral and faunal remains from Organic unit 1 correlate well with other probable early-Middle Wisconsinan (Boutellier interval) sites, most notably the Foraker Slump site in Alaska. Preliminary analysis indicates the presence of a previously unidentified type II tephra within Organic Unit 1, tentatively named Mariposa Creek tephra. Overlying Organic Unit 1 is a ~1m thick unit of weathered colluvium cut by an ice-wedge pseudomorph. This colluvium unit may represent severe conditions of the late Wisconsinan. Organic Unit 2 is Holocene with two distinct peat beds occurring near the bottom and top of the unit. Paleoecology of the upper peat (~2850 B.P.) suggests a mature spruce forest analogous to the modern ecological setting. Early Holocene samples from the lower peat bed suggest the presence of an open forest-shrub/tundra environment that was at least as cold and dry as the environment represented in Organic Unit 1.