DETRITAL ZIRCON U-PB AGES PROVIDE CONSTRAINTS ON THE DEPOSITIONAL AGE AND STRUCTURE OF THE FRANCISCAN COMPLEX, MT. TAMALPAIS, CA

The Franciscan Complex of California is considered by many to be the classic example of a subduction accretionary complex, yet few geochronologic studies have been undertaken on the Franciscan in the San Francisco Bay Area. Recent geologic mapping by our co-author (Bero) has identified several thick stratigraphic units consisting mostly of feldspathic arenite and minor serpentine exposed on the western flank of Mt. Tamalpais. Detrital zircon U-Pb ages from these sandstone strata help us constrain the maximum depositional age of the units and provide information on their provenance. The data also aid in correlating the units and help to determine the underlying structural architecture of Mt. Tamalpais.

Samples were collected along a west to east transect from just above sea level near the base of Mt. Tamalpais, to an elevation of approximately 2000 feet. Ten sandstone samples were collected consisting mainly of feldspathic arenite. The samples were separated at Sonoma State University and the detrital zircons were analyzed for U-Pb ages via LA-ICPMS at the University of Arizona LaserChron center. 120 to 240 grains were analyzed for each sample yielding a total of 1158 detrital zircon ages used for comparison and correlation of the strata.

The U-Pb ages indicate the strata have maximum depositional ages ranging from ca. 87 Ma to ca. 120 Ma. Near the base of the section, the depositional ages appear to be relatively consistent at ca. 87 Ma with primary age groups of 87-100 Ma, 105-120 Ma, 135-175 Ma, 185 to 220 Ma. The lower units also have a number of Proterozoic ages with age peaks at ca. 1370 and ca. 1750. Strata in the upper part of Mt. Tamalpais show more variability compared to the basal units and have depositional ages ranging from ca. 108 Ma to ca. 120 Ma. The primary age groups of the upper samples typically have ages ranging from 110-170 Ma, with age peaks commonly at ca. 120 Ma and ca. 140 Ma and very few Proterozoic and Archean ages. These data are consistent with a primary source region within the Sierra Nevada Arc. The obtained detrital zircon ages suggest that although some internal stratigraphy exists within the section, that the strata must be fault bound as younger depositional ages occur near the base of the stratigraphic section and older depositional ages occur near the top of the section.