Heavy Mineral Assemblages in Some Tertiary Sediments in San Mateo County, California

Abstract

ABSTRACT Four heavy mineral assemblages, each characteristic of a stratigraphic horizon, have been found in Eocene and Miocene sediments disclosed along the Stanford Linear Accelerator excavation in central California; they are: Middle and Upper Eocene: zircon-tourmaline Upper Eocene: biotite-garnet-sphene Lower Middle Miocene: epidote-glaucophane-chromite-hornblende Upper Middle Miocene: sphene-garnet-zircon-tourmaline. The Middle and Upper Eocene assemblage was derived from the Sierra Nevada, and it appears that this assemblage has a widespread occurrence in central California. Paucity of heavy minerals, a distinctive characteristic of this assemblage, and abundance of authigenic minerals are attributed to intensive weathering of source rocks and diagenetic alterations. The Upper Eocene assemblage was derived largely from intrusive and metamorphic rocks of the Coast Range. Predominance of epidote and abundance of chromite, hornblende, and glaucophane in the lower Middle Miocene assemblage indicate that during that time the area was situated in an enclosed embayment bordered by rocks of the Franciscan group. The upper Middle Miocene assemblage shows a mixed character, minerals characteristic of both he Sierra Nevada province and the Franciscan rocks of the Diablo and Coast Ranges are persistent constituents.