Clipper Gap Formation
Clipper Gap Formation
early Middle Jurassic to Late Triassic
Originally named by Lindgren (1900). Includes the olistostrome member of American River terrane of Graymer and Jones (1994). Equivalent to the southern part of the chert-argillite unit of Day and others (1985) and part of the Fiddle Creek complex of Edelman and others (1989). Cherty argillite and chert, with minor volcaniclastic sandstone, diamictite, breccia, and conglomerate. Includes large blocks and lenses of gabbro, chert, and marble. Late Triassic chert interbedded in the northern part of the unit (Hietanen, 1981) and early Middle to late Early Jurassic (Aalenian-Toarcian) chert interbedded in the southern part of the unit (Graymer and Jones, 1984). This unit also includes a lahar that has yielded a hornblende Ar-Ar date of ~174 Ma (Hacker, 1993). Lowermost greenschist facies. The formally accepted age of this unit is early Mesozoic and Mississippian, but the Mississippian age is based on fossils from limestone blocks (Lindgren, 1900) now seen to be olistoliths (Day and others, 1985) thus only providing a maximum age for the unit. We assign the age of this unit based on age of interbedded chert and volcanics.