Colfax Formation
Colfax Formation
Middle Jurassic, early Callovian
Slate, quartzose wacke, and conglomerate. Ammonites from this unit indicate an early Callovian age (Imlay, 1961). Chert blocks and pebbles from the conglomerate have yielded Early Jurassic (Toarcian-Pleinsbachian) and Late Triassic (Norian) radiolarians (Graymer, 1992; Graymer and Jones, 1994). Conglomerate includes clasts apparently derived from underlying Clipper Gap Formation (unit YAGC). The formally accepted age of this unit is Late Jurassic based on work in the early 20th century, but that is belied by the age of the fossils found later. Previously mapped as Mariposa Formation (e.g. Chandra, 1961), but these rocks are older.