OPS CHERT CONUNDRA IN FRANCISCAN COMPLEX ACCRETIONARY UNITS

Two chert conundra provide a challenge to understanding the accretionary complex architecture and history of the Franciscan Complex of California and southwestern Oregon. First, widespread chert sequences falling within the same broad age range (Early Jurassic – middle Cretaceous) are generally thought to have formed as part of the equatorial paleo-Pacific ocean OPS. The chert sections are now generally thought to be faulted, as exotic additions, into the dominant arc-derived wacke-mudrock sections of the Franciscan Complex. Many chronostratigraphically equivalent chert sections have been assigned to several different tectonostratigraphic terranes. Yet, if the chert sections are all fragments of the same single and areally extensive diachronous pelagic depositional unit, it violates the “rules” of terraneology for them to be assigned to different terranes. The second conundrum derives from the fact that over the past 100 years, many cherts have been described as being interlayered with sandstones in sedimentary sections, rather than being faulted into these sections. If interlayered chert-sandstone sections exist, (1) they demand expanded or alternative explanations of or for the traditional, singular basite-chert-sandstone+mudrock stratigraphy of the conceptual OPS, and (2) they require that stratigraphic chert-sandstone sections be distinguished from fault-bounded inter-terrane and intra-terrane tectonostratigraphic OPS units important in Franciscan terrane or accretionary unit evaluations. Earlier reported, well-known occurrences of, and more recently described, chert-sandstone associations are re-evaluated as a preliminary step towards the more extensive and needed re-evaluation of widely distributed chert sections across the Franciscan Complex. Some exposures clearly show that locally, chert layers constitute interlayers within commonly metamorphosed wacke-mudrock sections, reflecting chert deposition between temporally spaced turbidity current deposits derived from relatively nearer sources, versus the thick sections of OPS chert that represent long uninterrupted intervals of distant pelagic sedimentation. These chert sections require re-evaluation of sedimentation history and of unmapped conceptual faults “mapped” in the Franciscan architecture.