TEPHROCHRONOLOGY OF THE MODELO FORMATION, BALCOM CANYON, VENTURA COUNTY, CALIFORNIA

The Modelo Formation (Modelo) has long been recognized as an upper Miocene siliceous sedimentary unit composed primarily of sandstone and diatomaceous shale with interbedded tuffs. However, the numerical ages for the Modelo are sparse. Although over 30 eruptions of the Yellowstone hotspot occurred between 23 Ma and 5 Ma, with tuffs from these eruptions identified by tephrochronology as far south as the northern Mojave Desert 700 km from the source, there are no published tephrochronology results from the Modelo. In Balcom Canyon, southeast of Santa Paula, California, two vitric tuffs were collected from the Modelo. The glass shards from these samples were analyzed using electronprobe microanalysis (EPMA) to determine their major- and minor- element compositions and compared to over 7000 analyses in the U.S. Geological Survey tephra database. The older tuff (PRM-BC-2) correlates with the 8.99 Ma McMullen Creek tuff found in Trapper Creek, Idaho, and a tuff found in the Monterey Formation at Dos Pueblos Beach in Santa Barbara, California. The younger tuff (PRM-BC-1) correlates with another tuff in the Dos Pueblos Beach section. These correlations are consistent with published micropaleontology that indicates that the Balcom Canyon section is upper Mohnian. The relatively high iron content (3.13 and 2.58 wt% Fe2O3, respectively) is consistent with the geochemical composition of glass shards erupted from the Yellowstone hotspot. In the present geography, our results indicate that the air fall distribution of the McMullen Creek tuff was at least 900 km from the source; that distance is greater if estimated tectonic reconstructions that place the Transverse Ranges farther south during the Miocene are correct. This suggests that the McMullen Creek eruption was of similar magnitude to the better-known Lava Creek B and Huckleberry Ridge eruptions.