THE WHEATFIELD FORK TERRANE: A REMNANT OF SILETZIA (?) IN FRANCISCAN COMPLEX COASTAL BELT OF NORTHERN CALIFORNIA

A narrow, sheared fault sliver of Franciscan Complex east of Gualala contrasts with other Coastal Belt rocks in being the only mafic Eocene volcanic rocks and pelagic limestone of oceanic affinity known to the California accretionary margin and Franciscan Complex. The 13 km-long, northwest-trending sliver consists of a basal unit of basaltic flows and breccias with intercalated pelagic foraminiferal limestone overlain by arkosic wacke. Like the Coastal Belt as a whole, these rocks are veined with calcite and laumontite. We name these rocks the Wheatfield Fork terrane (WFT) herein, for their occurrence along the upper Wheatfield Fork of the Gualala River, where they crop out beneath the Coastal Belt Thrust, which separates the Coastal Belt below, from the structurally overlying Central Belt of the Franciscan Complex. The WFT contains pelagic limestone with a late early to middle Eocene (ca. 51- 46 Ma) foraminiferal fauna that is identical in age and depositional setting to limestone in the lower Crescent Volcanics of the Siletz terrane of the Pacific Northwest. Other basaltic basement rocks and associated overlying pelagic limestone in the Coastal Belt are of Late Cretaceous age (Campanian-Maestrichtean) and occur W-NW and outboard of the WFT. Foraminifer affinities, paleomagnetic data from the limestone, and basalt geochemistry collectively suggest that the Cretaceous basaltic rocks are part of the Farallon plate and formed ca. 82-70 Ma near the Pacific-Farallon ridge (ca. 20°- 28° N paleolatitude) and then were translated northeastward to interact with the California margin in the Paleocene to late middle Eocene (ca. 56-40 Ma). The unique age of the WFT within the Franciscan Complex and its oceanic character lead us to propose its correlation instead, with rocks of the Siletz terrane. The WFT may have formed with Siletz rocks on a southeastern part of the Kula (or Resurrection?) plate prior to or during Kula-Farallon ridge demise (by ca. 47 Ma), received sediment from the continental margin soon thereafter, and finally accreted along the inboard side of the Coastal Belt by the early Miocene (ca. 22-24 Ma). If so, then a part of the Kula-Farallon ridge system (southern option) must have been offset significantly southeast of the inferred ridge trend of published models.