Nd and Sr isotopic data are presented for argillaceous rocks from two terranes in the Klamath Mountains: the western Klamath terrane and Rattlesnake Creek terrane. In the collage of terranes exposed in the Klamath Mountains, these terranes are located farthest outboard from the North American craton and include (meta)igneous rocks of clear oceanic affinity. Nevertheless, the argillaceous rocks from these terranes preserve a strong isotopic signal of terrigenous sedimentary input. The lowermost portion of the Rattlesnake Creek terrane, the serpentinite-matrix mélange, is interpreted to have formed within an oceanic fracture zone. The argillaceous sediment incorporated into this tectonic assemblage was probably derived partly from eolian dust eroded from the continents and partly from local juvenile detritus shed from topographic highs along the fracture zone. The Upper Triassic–Lower Jurassic “cover sequence” of the Rattlesnake Creek terrane has been interpreted as an oceanic-arc assemblage, but the argillaceous rocks of the cover sequence have the most negative ϵ_Nd (−8.3) and radiogenic ⁸⁷Sr/⁸⁶Sr (0.7114) of any samples analyzed in this study. We infer that cratonic sediment was delivered to the depocenter of the Rattlesnake Creek terrane arc, probably transported by river systems. This situation suggests proximity to a continental landmass during arc magmatism. The Galice Formation, a thick turbiditic sequence above the Late Jurassic Josephine ophiolite, appears to be composed of detritus shed from both the Rogue–Chetco oceanic arc on the west (in present geographic coordinates) and previously accreted Klamath Mountains terranes and/or North American craton to the east. The continental isotopic signal is stronger in the argillaceous rocks than in the (meta)graywackes, suggesting that the finer-grained rocks contain a greater proportion of cratonic debris, material that may have been reduced to mud-sized particles during sediment recycling. The presence of continental-derived sediment in these otherwise ensimatic terranes indicates that although continental crustal growth by accretion of oceanic terranes is an important process, such accreted terranes commonly are not composed entirely of juvenile crust.