DETRITAL ZIRCON CONSTRAINTS ON SEDIMENT DISTRIBUTION AND PROVENANCE OF THE MARIPOSA FORMATION, CENTRAL SIERRA NEVADA FOOTHILLS, CALIFORNIA

New single-grain detrital zircon age data from sandstone lenses in the Upper Jurassic Mariposa Formation of the Foothills metamorphic belt indicate that: (1) the earliest phase of clastic sedimentation mainly involved material derived from the Eastern Klamath terrane and the Nevadan miogeocline, with modest input from the Sierra Nevada arc; (2) the arc became the dominant sediment source for the upper turbidite interval; and (3) the youngest zircon ages constrain the onset of clastic deposition between 153-151 Ma. The zircon data also suggest that the local drainage divide migrated westward over time, resulting in a higher proportion of detritus derived from the Sierra Nevada arc. New geologic mapping in the central Sierra Nevada Foothills shows that the Mariposa Formation thickens to the east, and that the number of coarse-grained sandstone bodies increases up section. These observations suggest that a topographically low Sierran volcanic arc gradually began to inflate, providing increasing amounts of clastic debris to the Mariposa basin.

The Mariposa Formation was deposited in a volcanically active deep-water forearc basin, subsequently disrupted by Late Jurassic Nevadan orogenesis. Inasmuch as it was located in the forearc, inboard of the Middle Jurassic Coast Range Ophiolite, Nevadan deformation cannot have been the product of an arc collision in the Sierra Nevada Foothills, but instead must be related to processes occurring along the plate margin.