BATHYSIPHON (FORAMINIFERIDA) AT PACHECO PASS, CALIFORNIA: A GEOPETAL, PALEOBATHEMETRIC AND PALEOCURRENT INDICATOR IN THE FRANCISCAN FORMATION

The turbidite beds of the Franciscan Complex are typically nondescript with few distinct features for correlating stratigraphy and structures in the field. In the Coast Ranges of Central California, numerous outcrops of the Franciscan Formation are exposed along Highway 152 and the San Luis Reservoir in Pacheco Pass.

The aggulinating foraminiferid Bathisiphon occurs in several outcrops throughout the eastern side of the pass. Bathysiphon is one of the largest single-celled organisms known. It survives by filter feeding in quiet cold-water environments such as deep-sea trenches. These rocks are known to have been metamorphosed to jadeite-grade (150º C, 7 kb), but these fossils show minor distortion or alteration.

Stratigraphic and sedimentologic analyses of the outcrops in Pacheco Pass suggest a distal slope fan as the environment of deposition. The preserved remains of Bathysiphon at Pacheco Pass measure up to 153 mm in length, and can be easily recognized in outcrop. In hand specimen and thin section, they are nearly featureless, silicified tubes that occur in the tops of hemipelagic beds. These Bathysiphon lie in the plane of bedding and have a dispersal pattern that appears to align with the subduction-trench axes. Bathysiphon have the potential to be useful geopetal, paleobathemetric and paleocurrent indicators for field mapping in the Franciscan Formation.