Allochthonous Carbonate Debris in Mesozoic Flysch Deposits in Santa Ana Mountains, California

Abstract

The Bedford Canyon Formation, in the Santa Ana Mountains, California, is predominantly a flysch-type sequence of alternating turbidite sandstones and pelagic shales. Within the dominant flysch sequence, however, are allochthonous limestone bodies, from 3 to 75 m long and 1.5 to 18 m thick. These limestones are commonly in abrupt contact with pebbly mudstones inferred to have been deposited primarily from submarine mass flows. The allochthonous limestones consist predominantly of clasts of shallow- and deep-water origin juxtaposed in a pervasive lime-mud matrix, which also includes both shallow-water crinoid and deep-water ammonite fossils. The carbonate debris was transported by submarine mass flows and was deposited essentially as thick units similar to the manner in which subaerial debris flows are deposited. Periods of deposition of the carbonate debris were short-term events during the long-term deposition of the surrounding detrital turbidite sands and pelagic shales. The Jurassic (Bajocian to Callovian) age of the allochthonous fauna contained in the Bedford Canyon limestones represents a maximum age limit for the allochthonous carbonate debris and, therefore, may not reflect the depositional age of the dominant sandstone-shale sequence. Consequently, the age of the Bedford Canyon Formation and its correlation with the Santa Monica Slate are uncertain. However, allogenic carbonate constituents in the detrital turbidites demonstrate that a shallow-water carbonate environment was present when the detrital sands and shales were forming. From stratigraphic relations, the youngest rocks of the Bedford Canyon Formation may be Albian (late Early Cretaceous) in age. In view of this interpretation and the strong lithologic similarities with Franciscan rocks, the Bedford Canyon Formation appears to represent a southern extension of the Franciscan Complex, which crops out predominantly north of the Transverse Ranges. The presence of a Mesozoic carbonate shelf, as demonstrated by the provenance of the Bedford Canyon limestones, represents a new paleogeographic province in southern California.