Tectonostratigraphic significance of sedimentary strata occurring within and above the Coast Range Ophiolite (California Coast Ranges) and the Josephine Ophiolite (Klamath Mountains), northwestern California

Abstract

In the study of ophiolites, an analysis of sedimentary rock that formed as interpillow sediment and as strata that rest in depositional contact above the lava can supply valuable data pertaining to ophiolite tectonogenesis. This paper focuses on two North American ophiolites: The Coast Range ophiolite in the California Coast Ranges and the Josephine ophiolite in the Klamath Mountains. The Coast Range ophiolite's lithosphere formed at an open-ocean spreading center where calcareous pelagic sedimentation and ferruginous and siliceous hydrothermal deposition accompanied volcanism. On the basis of an analysis of the physical stratigraphy, the geochronometry, and the radiolarian biostratigraphy, we conclude that a disconformity representing an 8 to 11 m.y. hiatus occurs between the Coast Range ophiolite and the overlying volcanopelagic succession. This unconformity is correlative chronostratigraphically with that occurring in the San Pedro del Gallo terrane southwest of the Walper megashear in Mexico and in western Cuba. Therefore, we postulate that the regional unconformity indirectly reflects the titanic forces at work during the final breakup of Pangea and the opening up of the North Atlantic. Radiolarian faunal and paleomagnetic data show that all remnants of the Coast Range ophiolite originated at near-equatorial paleolatitudes (Central Tethyan province) during the latest Middle Jurassic and were rapidly displaced northward to higher paleolatitudes (Northern Tethyan and Southern Boreal provinces) during the Late Jurassic. During northward tectonic transport, the Coast Range ophiolite plate subsided to abyssal depths and traversed a region of nondeposition (starved sedimentation, sediment bypassing, erosion) during the late Bathonian to early Oxfordian. It then moved through a region of siliceous (radiolarian) pelagic and tuffaceous volcaniclastic sedimentation-the depositional apron from an adjacent active volcanic arc-during the middle Oxfordian, Kimmeridgian, and Tithonian; the resulting deposits are termed volcanopelagic. The contact between the volcanopelagic sedimentary rocks and the Great Valley Supergroup marks a period in which waning volcanogenic sedimentation was overwhelmed by a sudden influx of voluminous siliciclastic turbidite that originated from the continental margin (the uplifted Jurassic volcanoplutonic arc) at the beginning of the Nevadan orogeny (late Tithonian: lower Subzone 4α [based on Radiolaria]). A period of turbidite deposition occurred at precisely the same time and biohorizon in the distal backarc domain (San Pedro del Gallo terrane) in Mexico (age substantiated by ammonites, Buchia, and Radiolaria). Moreover, paleolatitudinal (faunal and paleomagnetic) data place these San Pedro del Gallo terrane remnants at approximately the same latitude as the Jurassic volcanoplutonic arc (Sierra Nevada segment) during the Late Jurassic (late Tithonian). The Josephine ophiolite formed at a backarc or forearc spreading center close to a Jurassic volcanoplutonic arc where calc-alkalic interpillow tuff and tuffaceous chert occur within the volcanic member of the ophiolite. In the Smith River subterrane, the Josephine ophiolite (162 ′ 1 Ma: late Callovian) is conformably overlain by volcanopelagic strata. Continuous volcanogenic sedimentation occurred during the late Callovian to early Oxfordian (? early middle Oxfordian). A low-latitude (Central Tethyan province) origin, coupled with northward transport, is also indicated by paleontological data (Radiolaria, ammonites, and Buchia) for the Jurassic Josephine ophiolite (Western Klamath terrane). The disconformable contact between the volcanopelagic and the Galice Formation (s.l.) reflects a sudden influx of siliciclastic turbidite from a Jurassic volcanoplutonic arc source area and from older rocks of the accreted continental margin that lay inboard of the arc. The same event is represented in the Foothills terrane (Sierra Nevada) by the deposition of the sil