A case of marginal palynology: A study of the Franciscan Melanges

Abstract

The Franciscan rocks on which much of greater San Francisco, California, is built, have been the subject of geologic investigation and speculation for a century. Recent investigations by Hsü and colleagues have indicated that type Franciscan rocks are mélanges, consisting of allochthonous blocks of diverse sedimentary units mixed by complex tectonic activity. Establishment of the original stratigraphic positions of the blocks has been hampered both by disruption of original continuity and scarcity of fossils. Twelve samples of somewhat metamorphosed shales and graywacke‐siltstones provided by Hsü from critical parts of his proposed tectonic plates were investigated for palynomorphs. Poorly preserved plant microfossils were obtained from two siltstone samples. The floras obtained show that one of the subunits investigated is Late Cretaceous in age. Another unit, for which an age of Late Jurassic has previously been suggested on rare occurrences of invertebrates, is shown on the basis of palynomorphs to be Late Jurassic to Early Cretaceous. This study of Franciscan mélanges illustrates the application of palynological methods to critical rock units in which palynomorphs are poorly preserved.