UNDERSTANDING THE “FRANCISCAN COMPLEX”

W. Porter Irwin

A curiously heterogeneous assemblage of rocks found on the San Francisco Peninsula and areas both north and south was long an enigmatic puzzle to geologists. Variously called the "Franciscan Formation," "Franciscan Series," "Franciscan Group," "Franciscan assemblage," or "Franciscan Complex," these rocks include deep-sea cherts, greywacke sandstones, altered mafic volcanic rocks (greenstones), shales, serpentinites, limestones, and high-pressure metamorphic rocks, all of them faulted and mixed in a sometimes seemingly chaotic manner. This assemblage of rocks was found to be the most widespread major component of the Coast Ranges of California. By the mid-1950’s, the Franciscan was recognized as a "eugeosynclinal" assemblage, but what that meant, beyond being a label, was not clearly understood. Nor was the relation of the Franciscan to the rest of California geology understood.

By 1957, reconnaissance mapping of much of the northern Coast Ranges and Klamath Mountains of California had established that the Franciscan Complex was roughly the same age as the adjacent Jurassic and Cretaceous Great Valley sequence, although the two units were recognized as being strikingly different in overall lithology and tectonic deformation. The Great Valley sequence, dominantly clastic sedimentary strata, is locally faulted into the Franciscan Complex, and in some instances the rocks of the Great Valley sequence and those of the Franciscan are hard to distinguish. To help resolve this problem, the sandstones of both units were extensively sampled for their potash feldspar content. The samples of the Great Valley sequence were found to generally contain a substantial percentage of potash feldspar, which increased with decreasing age of deposition, but samples of the Franciscan sandstone generally contained at most a trace of potash feldspar. (Some sandstones in a coastal belt contained substantial potash feldspar and were later found to be Tertiary in age, younger than typical Franciscan.)

These results and others were presented by Edgar H. Bailey, William P. Irwin, and David L. Jones, all USGS scientists based in Menlo Park, in the comprehensive report Franciscan and related rocks, and their significance in the geology of western California (California Division of Mines and Bulletin 183, 1964). That report, which summarized all that was then known about the Franciscan, was for several decades a popular textbook and reference and served to guide further research.

In the mid-1960's the European term "melange" was applied to chaotically deformed parts of the Franciscan. The advent of the plate tectonic concept in the late 1960’s created new interest and new interpretations for the development of the Franciscan. In the light of this new concept, the Franciscan came to be seen as an assemblage of oceanic crustal rocks and oceanic and continental margin sediments that had been incorporated into the continental margin by convergent plate motions. The presence of blueschist-facies metamorphic rocks, some of the intense deformation, and other features of the Franciscan could now be seen as products of subduction.

Development of the concept of accretionary terranes in the 1970’s furthered our understanding of the Franciscan and of the geology of the California Coast Ranges. Increased geologic quadrangle mapping and other studies have been invaluable for unraveling the tectonic evolution of the region. The Coast Ranges of northern California now are thought to represent a late Mesozoic zone of convergence of oceanic and North American tectonic plates, with the present distribution of the Franciscan and other rocks being greatly complicated during Cenozoic time by fragmentation and large amounts of right-lateral slippage along the San Andreas and numerous other faults along the boundaries between the plates. The Franciscan, though fascinating still, is no longer an enigma.

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