Paleogene strata of the Franciscan Coastal belt and the Yager complex (northern California) provide a vivid illustration of the problems encountered during studies of detrital provenance along accretionary continental margins. The northern portion of the Yager complex contains lower percentages of total quartz and higher percentages of feldspar than do correlative stratigraphic units to the south; mean Q-F-L modes are Q-30, F-54, L-16 (N-Yager) and Q-49, F-31, L-20 (S-Yager). A similar spatial trend is evident within the Coastal belt, where mean Q-F-L modes are Q-33, F-47, L-20 (N-CB) and Q-48, F-33, L-19 (S-CB). A third Coastal belt petrofacies (L-CB) is dominated by volcanic-lithic grains.
Interpretations of generic provenance for each petrofacies are straightforward. The arkosic sands probably were eroded from deeply dissected granitic plutons, and the more-quartzose sands are attributed to recycling from both orogenic backarc terranes and an adjacent magmatic arc. The influx of volcanic detritus proves that magmatism was active within the drainage basin during Paleogene time. Identification of specific source areas is more problematic, however. Perhaps the north-to-south changes in detrital mineralogy were caused by influx from an Idaho batholith source (arkosic) versus a Sierra Nevada source (quartzose). The depositional paleolatitude of Franciscan sands is unknown, and, in the absence of latitudinal control, potential sources as far south as Mexico remain plausible. Regardless of these uncertainties, the variations common to both Coastal and Yager terranes demonstrate that amalgamation must have preceded any translational events, such that differential slip has not occurred between the two terranes.