Abstract

A thick sequence of early to middle Miocene conglomerate and breccia on Santa Cruz Island is assigned to the Vaqueros Formation, the San Onofre Breccia, and the Blanca Formation. The Vaqueros and San Onofre intertongue with finer grained lithofacies on Santa Rosa and San Miguel Islands, and the Blanca Formation is recognized on all three islands. Each of the three units is characterized by a different suite of clast lithology, and the sequence of changes in clast composition probably reflects a sequence of tectonic events in the source terrane.

Uplift and erosion of local basement rocks are indicated by clasts of diorite and greenschist in the Vaqueros Formation conglomerate, the lowest unit of the conglomerate-breccia sequence. The overlying San Onofre Breccia includes abundant blueschist detritus derived from a Catalina Schist meta-morphic terrane. Dacitic volcanic clasts occur first at the base of the Blanca Formation, distinguishing it from the underlying San Onofre. They were probably derived from several volcanic events that were contributing andesitic and dacitic ejecta at that time from an area between Santa Cruz and San Miguel Islands.

Basin and ridge topography, with erosion of uplifted crystalline basement rocks, apparently began as early as Vaqueros time in the northern Channel Islands region.