Stratigraphy of the Fort Selkirk Volcanogenic Complex in central Yukon and its paleoclimatic significance: Ar/Ar and paleomagnetic data

Abstract

Brunhes, Matuyama, Kaena, and Mammoth age basaltic lava flows (Tertiary–Quaternary Selkirk Volcanics) were sampled in west-central Yukon. The mean characteristic remanent magnetization (ChRM) direction of the flows sampled in this and previous studies has a declination of 348.7° and an inclination of 70.8° (n = 42, k = 99.6, α95 = 2.2°) (all on lower hemisphere). The time range represented in this study (ca. 3.25 to ca. 0.004 Ma) is great enough to have confidently averaged secular variation. Sediment associated with the basalt has a mean declination of 7.6° and inclination of 78.8° (n = 5, k = 5.6, α95 = 35.7°). A new 40Ar–39Ar date on the reversely magnetized basal basalts at Ne Ch’e Ddhäwa places the eruption in the Mammoth subchron of the Gauss Normal Chron. The newly dated basal basalt at Ne Ch’e Ddhäwa precedes the initial continental glaciation in Yukon and is older than the Fort Selkirk vent (Lower Mushroom), which was previously thought to be the oldest eruption at Fort Selkirk Volcanic Complex (FSVC). This basal flow at Mushroom is dated at 1.82 ± 0.03 Ma and the uppermost flow is reproducibly dated at 1.36 ± 0.04 Ma. Till on the flanks of a subglacial volcanic mound called Ne Ch’e Ddhäwa (informal) is older than previously thought; its reverse magnetization indicates an Early Pleistocene age rather than the Reid glaciation, which falls during the Brunhes Normal Chron. The paleomagnetism of Tertiary–Quaternary Selkirk Volcanics outcrops outside the FSVC was studied for the first time. The ChRM direction of basalt at the northern edge of the northern Cordillera volcanic province agrees with FSVC directions, suggesting that this flow reflects the same period of volcanism. This suggests that an Eocene K–Ar date, previously thought to be unreliable, may well be correct.