UNMAPPED OCCURRENCE OF CHIASTOLITE SCHIST IN WESTERN MARIPOSA COUNTY, CALIFORNIA, EXTENDING THE RANGE OF METAMORPHISM FOR INCLUSION IN SMALL SCALE GEOLOGIC MAPS OF CALIFORNIA

The Chiastolite variety of Andalusite is identified by dark colored inclusions of carbonaceous material making a distinctive cross-shaped figure in cross section. Andalusite is typically formed in the contact aureole of igneous intrusions into argillaceous rocks. Our study is to better resolve the extent of this metamorphic locale, with the goal of including this as an update to future geologic maps of the area.

The location we studied is currently included on the Geologic Map of California (Jennings, 1977, updated in 2010 by Gutierrez, Bryant, Saucedo and Wills). Previous workers (Taliaferro et.al., 1930, Best, 1961, Bortugno, 1983, Wagner et.al., 1991) map our location as Upper Jurassic-Lower Cretaceous marine sandstone and shale, minor conglomerate, chert, slate, limestone, and minor pyroclastics. The region mapped is approximately one mile wide and 6 miles long. Other occurrences reported include Turner (1892) who reported chiastolite, sillimanite and mica; and Butler and Gale (1912) identify Tres Cerritos Buttes, approximately 1-2 km northwest of our site as containing Chiastolite associated with meta-greenstone and metamorphosed augite-andesite tuff.

Our study extends this region of metamorphism by 30 kilometers to the south east, with the farthest occurrence outcropping at Deadman Creek in Mariposa County, California.

The majority of the field work was completed at Bear Creek California, with other investigations at Owens Creek, Mariposa Creek, and Deadman Creek.

We collected over sixty orientations of the foliations in outcrops along a property that has, according to the property owner, not been studied in 150 years. Samples were collected for further optical and geochemical study.

Our findings increase the extent of the metamorphic range, in area, to the scale that inclusion in future maps is possible.