The Upper Triassic to Middle Jurassic Whitehorse Trough is a 500-km-long forearc basin developed on the eastern margin of the Stikine Terrane. Basin fill in the Fish Lake syncline consists of Early to Middle Jurassic strata of the Laberge Group. Re-examined fossil collections record all ammonite zones from the Upper Pliensbachian to lowest Bajocian (except for the recessive or volcanic Aalenian Stage), suggesting that the Laberge Group is a conformable sequence. Biostratigraphic dating of oldest conglomerates indicates that source area uplift occurred first in the north in the Early Pliensbachian, progressing southwards through the Late Pliensbachian. Periodic, basin-wide anoxia was prevalent during the Middle and Late Toarcian, indicated by abundant Bositra, a low-oxygen-tolerant bivalve. The final stage of basin-fill, Early Bajocian chert-pebble conglomerates, provides a maximum age constraint on the amalgamation of the Cache Creek and Stikine terranes.
