VOLCANISM AND AGE CONSTRAINTS ON PLIOCENE AND EARLY QUATERNARY CORDILLERAN ICE SHEET ADVANCES, FORT SELKIRK AREA, WEST-CENTRAL YUKON, CANADA

Fort Selkirk (FS) Yukon, Canada, is situated within (50 km east) of the all-time-limit of Cordilleran-ice-sheet glaciation (late Pliocene in age). Late Pliocene and early Pleistocene mafic volcanic rocks in the FS area are locally interstratified with glacial and non-glacial sediments and were erupted in part beneath late Pliocene and early Pleistocene Cordilleran ice sheets.

Neogene volcanism began in the Wolverine Creek basin immediately south of FS ca. 4.3 Ma and persisted to ca. 3.0 Ma. The Yukon River valley was filled with at least 40 m of lava flows by these eruptions. Volcanic activity at the Ne Ch’e Ddhäwa (NCD) eruptive center began with a subaqueous eruption of hyaloclastite tuff between ca. 3.0 and 3.1 Ma. It caused or was coincident with temporary volcanic damming of Yukon River.

The first evidence of incursion of a Cordilleran ice sheet into the FS area was coincident with a second eruption of NCD eruptive center. It built the NCD volcanic edifice beneath 300 m of glacial ice. Radiometric dating of hyaloclastites and their reversed remanent magnetism refer this eruption to marine isotope stage (MIS) 82 or 78. The subsequent Fort Selkirk Glaciation (FSG) is documented by till and outwash. These overlie complexes of magnetically reversed lava flows (ca. 1.8 Ma) from the FS eruptive center and underlie ca. 1.3 Ma lava flows from the Pelly eruptive center (PC) north of FS. The glacial sediments also underlie non glacial sediments that preserve the short (magnetically normal) Gilsa Subchron. Consequently, FSG occurred during MIS 62 or 60 or 58.

Eruption of PC dammed Yukon River with 70 m of lava at ca. 1.3 Ma. Striations and glacial erratics on these lava flows and reversely magnetized till document a glacial advance that reached FS during the last 500 kyr of the Matuyama Chron (MIS 40 to 20). ‘Forks Glaciation’ (FG) is proposed for this event.

The NCD and FS glaciations clearly occurred during MIS paced by the 41 kyr precession cycle whereas FG occurred during the transition between precession-paced and eccentricity-paced (100 kyr) glacial cycles. Glaciers never reached FS following FG. This pattern is in accord with the view that the progressive western expansion of North American continental ice sheets during eccentricity-paced glacial cycles resulted in progressively less moisture reaching the northern Cordillera.