NATURE AND U-PB ZIRCON AGES OF MID-CRETACEOUS CALDERAS AND TUFFS IN EASTERN ALASKA AND WESTERN YUKON: IMPLICATIONS FOR LANDSCAPE EVOLUTION IN THE NORTHERN CORDILLERA

The Sixtymile Butte (~170 km²) and Dennison Fork (~90 km²) calderas in central Tanacross quadrangle in E Alaska comprise thick (>400 m) accumulations of variably welded rhyolitic tuff (Bacon et al., JGR 1990) that yield preliminary laser ablation (LA) U-Pb zircon ages of 110.2 ± 0.5 and 107.7 ± 0.8 Ma, respectively. Thin felsic ash layers recognized in several localities in W Yukon may represent distal ash related to these calderas or to other coeval eruptive centers. A thin, densely welded ignimbrite unit that lies depositionally on a metamorphic basement ridgetop in the SW Dawson map area in W Yukon gives a LA U-Pb zircon age of 107.7 ± 0.9 Ma. This ignimbrite locality is >100 km from the calderas in E Alaska and is more likely to be related to an as yet unidentified, more proximal source. Thin felsic ash layers that are interbedded with coal-bearing sediments at two localities at the base of the mid- to Late Cretaceous Indian River Formation (IRF) conglomerate in W Stewart River map area in W Yukon give LA U-Pb zircon ages of 107.5 ± 0.8 and 108.0 ± 1.0 Ma. The IRF is a dominantly alluvial-fluvial pebble and cobble conglomerate unit as much as 500 m thick that is interpreted to have filled a local basin in western Yukon. Although the IRF conglomerate consists almost entirely of material eroded from the underlying metamorphic basement, rounded clasts of quartz-feldspar porphyry are also present and locally abundant near the base of the unit. These clasts likely represent material eroded from exhumed subvolcanic intrusions that may have underlain mid-Cretaceous calderas or other felsic volcanic centers in the region (e.g., the ~108 Ma Dawson Range batholith or the ~110 Ma Crag pluton, both in W Yukon). Results reported here help constrain the paleotopography of W Yukon and E Alaska. No outflow tuff from the Tanacross calderas has been identified thus far and, given the possible thickness of caldera fill, the present erosional surface may have been as much as 4 km beneath the mid-Cretaceous surface. Nonetheless, the presence of these calderas and coeval distal ash deposits helps constrain the nature and position of the ~110 Ma paleosurface across a large region in the northern Cordillera. The mid-Cretaceous surface must have had significant relief in order to explain the presence of the IRF basin.