A 2.5 MA CHRONOLOGY OF REGIONAL GLACIATION IN WEST-CENTRAL YUKON, CANADA BASED ON RADIOMETRIC AND PALEOMAGNETIC DATING OF VOLCANIC ROCKS

Since the Late Pliocene, the Yukon Plateaus of west-central Yukon Territory, Canada, have experienced periodic fissure eruptions of valley-filling mafic lavas and eruption of small volcanoes (Selkirk Volcanics Group). Ar-Ar and fission track dating as well as paleomagnetic investigations of these volcanic rocks and interstratified sediments chronologically bracket at least three regional glaciations from the Late Pliocene through the Early Pleistocene. The ancestral valley of Rosebud Creek (63° 15’ N, 137° 18’ W) contains a magnetically normal basalt flow Ar-Ar dated at 2.69 +/? 0.04 Ma. The valley was buried by thick ice-terminus drift deposited during the most extensive and likely the oldest regional glaciation. Regional geomorphic evidence suggests that this glaciation reversed the ancestral Yukon River. The Yukon River assumed its present course by 1.83 +/-0.3 Ma when a magnetically reversed basanite flow was erupted near Fort Selkirk (62° 45’ N, 137° 25’ W). The next Early Pleistocene regional glaciation (Fort Selkirk Glaciation (FSG)) is documented by a till near Fort Selkirk. The till is bracketed in time by Ar-Ar ages on overlying and underlying basalt flows and overlying fission-track-dated tephra. These limit FSG to one of the cold peaks that occurred between oxygen isotope stages (OIS) 64 and 50 inclusive (ca 1.8 to 1.5 Ma). A subsequent regional glaciation left striations and erratics on magnetically reversed alkaline olivine basalt flows dated at 1.47 +/- 0.05 Ma. This glaciation predated the eruption of a magnetically normal complex of valley-filling olivine basalt flows and pillow basalts that dammed the Yukon River near Fort Selkirk 311 +/- 30 ka. The complex is overlain in part by outwash from an ice sheet that formed during Reid Glaciation. Limiting ages determined on tephra overlying Reid drift along with the Ar-Ar age determined on the underlying volcanic rocks date Reid Glaciation to OIS 8. No deposits from OIS 6 ice sheets have been found in Yukon. The OIS 2 (McConnell Glaciation) ice sheet terminated east of Selkirk Volcanics.