. Bulletin. Science; Natural history; Natural history. GEOLOGY OF CEDROS ISLAND 95 NW S E Cotoradito Choyal Eugenia Vallc Fm VaBe Fm Fm (Upi) (Lwt). Gran Canon | Fm. BORDER ) FAULT Section A- A 0 I z 3 km Fig. 4. Geological Section A-A'. Jurassic (Bajocian-Callovian) age (Imlay, pers. 1968). Similar lithologic sequences, which rest depositionally upon oceanic crust, are reported from the Coast Ranges (Bailey et al., 1970; Page, 1972), but these appear younger (, late Jurassic-early Cretaceous) than the Gran Caiion. Gran Canon correlatives may occur in the Sierra Nevada or Peninsular
. Bulletin. Science; Natural history; Natural history. GEOLOGY OF CEDROS ISLAND 95 NW S E Cotoradito Choyal Eugenia Vallc Fm VaBe Fm Fm (Upi) (Lwt). Gran Canon | Fm. BORDER ) FAULT Section A- A 0 I z 3 km Fig. 4. Geological Section A-A'. Jurassic (Bajocian-Callovian) age (Imlay, pers. 1968). Similar lithologic sequences, which rest depositionally upon oceanic crust, are reported from the Coast Ranges (Bailey et al., 1970; Page, 1972), but these appear younger (, late Jurassic-early Cretaceous) than the Gran Caiion. Gran Canon correlatives may occur in the Sierra Nevada or Peninsular Ranges which include middle-late Jurassic sedimentary sequences (Schweikert and Cowan. 1975; Imlay, 1964). Cedros Island Ophiolitc.—If the Choyal is correctly interpreted as an oceanic lithosphere remnant, then siliceous shale of the Gran Cation and the Choyal con- stitute an ophiolite. The ophiolite is not completely exposed at any one place be- cause of faulting and overlap by younger beds. The Choyal peridotite/serpentinite and gabbro/diorite belts are exposed 5 km west of Cedros town, while Choyal greenstones and Gran Caiion siliceous shale are revealed in Gran Canon. The San Carlos Fault and Subduction The San Carlos fault is a steeply dipping, northeasterly trending fault which separates Cedros and Choyal formations along the western and northern slopes of Monte Cedros. Along it. Cedros rocks are locally foliated and altered to glaucophane schist, while the adjacent Choyal serpentinite/peridotite belt is per- vasively sheared. The arcuate pattern of the Choyal. dipping steeply away from a Cedros ";' suggests that the Cedros Formation structurally underlies the Choyal. The absence of dikes in the Cedros, so widespread in the o\erl\ing Choyal, coupled with melange development within the Cedro^ Formation itself, suggests that the Cedros was tectonically emplaced beneath the Choyal, either by subduction of Cedros or obdiictiiMi of Cho\ al. CJiven substantial
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