Abstract

We present the result of biostratigraphic and lithologic studies of ribbon chert and associated rocks in a Mesozoic subduction complex in west-central Baja California, Mexico. The subduction complex terrane is divided into three subterranes, each of which consists of an originally coherent sequence of interbedded oceanic rocks, including ocean-floor basalt, terrigenous siliciclastic turbidites, radiolarian ribbon chert, and limestone. Parts of the original stratigraphy are preserved in the least deformed, least metamorphosed subterrane. Ocean-floor pillow basalt on Cedros Island is overlain by about 40 m of radiolarian ribbon chert ranging in age from Early Jurassic or older to Early Cretaceous; the chert is overlain by thin-bedded turbidites. On San Benito West Island, radiolarian ribbon chert is interbedded with volcanogenic rocks, including a megabreccia that contains blocks of metabasites, folded Late Jurassic-Early Cretaceous chert, and marble in a mid-Cretaceous radiolarite matrix. Chert samples from tectonically thinned sequences elsewhere on Cedros and San Benito Islands contain Late Triassic to Early Cretaceous radiolarian assemblages.

The 40-m-thick chert section on Cedros Island probably accumulated atop oceanic crust in a large ocean basin for at least 65 m.y. and was later capped by terrigenous turbidites as it approached the subduction zone along the western margin of North America. Faulted chert sections on Cedros and San Benito Central and West Islands are subsets of the main section on Cedros Island. The chert/megabreccia section on San Benito West Island is interpreted to have accumulated at the foot of a volcanic edifice on the ocean floor (for example, seamount, fracture zone) or during extension and normal faulting of the downgoing oceanic plate in a subduction zone. Sedimentary structures and stratigraphic relations indicate that all ribbon chert in this subduction complex was deposited by sediment gravity flows; that is, they are turbidites.

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