Ribbon cherts and siliceous tuffs of the North Fork and Rattlesnake Creek terranes of the Klamath Mountains yield Mesozoic radiolarians. Rocks of the North Fork terrane were previously considered to be of Paleozoic age and those of the Rattlesnake Creek to be of Paleozoic and Triassic age, on the basis of fossiliferous limestone bodies that are now considered to be exotic blocks. In both terranes, however, red cherts that are closely associated with ophiolitic rocks contain Late Triassic radiolarians; overlying cherts and siliceous tuffs contain Early or Middle Jurassic radiolarians. The Jurassic radiolarian fauna from the North Fork terrane is similar to a fauna contained in Franciscan chert near Santa Barbara in southern California.
The change in age assignment of the dominant rocks of these terranes, based on this new radiolarian data, indicates that the suture between the North Fork terrane and the Devonian rocks of the central metamorphic belt on the east probably formed during Middle or Late Jurassic time.