BIRTH OF THE NORTHERN CORDILLERAN OROGEN, AS RECORDED BY JURASSIC SEDIMENTATION AND EXHUMATION IN YUKON

Whitehorse trough (WT) is an Early to Middle Jurassic (late Hettangian-Bajocian) marine sedimentary basin that overlaps the Intermontane terranes in the northern Cordillera. In Yukon, the Laberge Gp. (= WT) comprises two lateral facies: 1) a southern unit of deep-water turbidite and mass-flow conglomerate, the Richthofen fm.; and 2) a northern unit of shallow-marine to fluvial sandstone, conglomerate and minor shale with coal, the Tanglefoot fm. Several horizons of pyroclastic crystal-lithic tuff (Nordenskiold unit) are common to both formations and dated at ca. 188-184 Ma. Upward change in clast content of Laberge Gp. conglomerate suggests progressive exhumation of adjacent Late Triassic-Early Jurassic arcs, with predominance of supracrustal volcanic and sedimentary clasts low in the section, and mainly plutonic clasts higher up. Detrital zircons from Laberge Gp. sandstones all show similar age distributions with the majority of grains in the 220-180 Ma range, and minor mid-Paleozoic ages that correspond exactly with known igneous ages from areas surrounding WT. In addition, source terrains for these zircon populations are generally characterized by Early to mid-Jurassic mica cooling ages (ca. 200-175 Ma) and the petrology of Early Jurassic granitoid plutons flanking WT suggests rapid exhumation during emplacement. Together, these data suggest that foundering of the WT and coarse clastic sedimentation occurred concurrently with rapid exhumation of the basin’s shoulders, to the east, north and west of the trough. Isolated occurrences of similar sandstone and conglomerate units with identical detrital zircon signatures are also documented ~200 km west and ~150 km east of WT, respectively, as well as overlapping Cache Creek terrane. These occurrences either indicate that WT was once more extensive or that these other occurrences developed as isolated smaller basins tapping similar sources. Development of these sedimentary basins and accompanying rapid exhumation in the northern Cordillera is coeval with documented onset of orogenic activity in the hinterland of the southern Canadian Cordillera. These basins are interpreted as piggyback and foreland basins developed atop the nascent orogen and their sediment record witnessed the birth of the Cordilleran orogen.