ABSTRACT

A total of 38 heavy minerals were found in Butano (Eocene), San Lorenzo (Oligocene) and Vaqueros (Miocene) Formations in the Santa Cruz Mountains, but of these, only 13 occur in appreciable amounts. They are: apatite, biotite, chlorite, epidote, garnet, hematite, ilmenite, leucoxene-anatase, pyrite, rutile, sphene, tourmaline, and zircon.

Stratigraphic variations of heavy mineral frequencies in the Butano and Vaqueros Formations are significant through both vertical and horizontal intervals. The San Lorenzo Formation, however, is dominantly shale, and the percentages of heavy minerals therein are too low to allow detailed study. The mineral variations in the Butano and Vaqueros sands are attributed to four main processes: (1) differential sorting, (2) concentration of stable minerals by removal of unstable species, (3) diagenetic alteration, and (4) changes in intensity of erosion and weathering at the source plus changes in mode of transport to the site of deposition.

Diagenesis, which plays an important role in the alteration of ilmenite and sphene to leucoxene (or anatase), appears to have been more active in the northernmost sections of these formations, where increased distance from shore and source of sediments may have been a controlling factor in mineral alterations.

The main distributive province from which the heavy minerals of these lower Tertiary sedimentary rocks were derived is the area in which the Ben Lomond quartz diorite and adjacent metamorphic rocks occur. A secondary terrane which appears to have made a minor but significant contribution to early Tertiary sedimentation in this area is the pre-Tertiary (Franciscan) sedimentary, metamorphic, and basic igneous terrane northeast of the San Andreas fault.