ESTIMATED VOLUME OF VOLCANIC ASH FALL: OLIGOCENE OTAY FM

Products of Tertiary subduction related volcanism have long been sought along the Southern California coast. Oligocene (28.86 Ma) volcanic activity in Southwestern San Diego County, California and North Eastern Baja California, Mexico contributed substantial volcanic ash to the Otay formation. Ash and derived waxy bentonite are described in Berry (1999) Eocene and Oligocene Otay-type Waxy Bentonites of San Diego County and Baja California: Chemistry, Mineralogy, Petrology and Plate Tectonic Implications: Clays and Clay Minerals, v 47, n 1. An estimate of the minimum volume of volcanic ash incorporated in the Otay formation was made by using measured sections throughout the area where exposures occur East of the La Nacion Fault. Claly sized constituents are estimated to be 10% of the total volume. Thin sections of waxy (pure volcanic origin) clay sized material showed no disceernible volume change between volcanic glass shards and I/S "ghosts" of shards. X-ray diffraction data show clay-sized fractions consist of smectite-rich I/S with minor amounts of illite and few if any other minerals. Variations in "d" spacings of the "~17Å" and "~8.6Å" peaks were used to discern how much I/S was derived from volcanic ash and how much was non-volcanic in origin. Deposits of waxy I/S were not included in the estimate other than as they contributed to the total volume of the Otay fm. According to the considerations listed, the minimum volume of volcanic ash was 114x10⁶ m³. Volume of the unexposed Otay formation West (downthrown side) of the La Nacion Fault is probably equal to or greater than that found East of the fault. If true, this would at least double the estimate of the minimum amount of volcanic ash that fell during deposition of the Otay formation. All Eocene and Oligocene formations in Southwest San Diego County and Northwest Baja California, Mexico show clear evidence of incorporating volcanic ash during their deposition thus further increasing the volume of Tertiary volcanic ash.