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

The Modelo Formation (Modelo) has long been recognized as an upper Miocene, siliceous-diatomaceous sedimentary unit with interbedded tuffs found in the California Transverse Ranges, north of the Malibu Coast fault. Marine biostratigraphy shows that the Modelo at Balcom Canyon is upper Mohnian (~9-7.5 Ma). The age of the Modelo overlaps with Yellowstone hotspot eruptions (ca. 23 to 5 Ma) from the Snake River Plain (SRP), southern Idaho region, which are found as far south as the northern Mojave Desert. Despite this temporal overlap and known tuffs, there are sparse numerical ages and no published tephrochronology results from the Modelo. Two vitric tuffs were collected from the Modelo section in Balcom Canyon, southeast of Santa Paula, California. The glass shards from these samples were analyzed using electronprobe microanalysis (EPMA) to determine their major- and minor- element compositions and compared to over 7,000 analyses in the U.S. Geological Survey (USGS) tephra database. Trace element compositions of the shards were also obtained using inductively-coupled-plasma mass spectrometry (ICP-MS). The first tuff (PRM-BC-1) correlates with the 8.99 Ma Tuff of McMullen Creek that erupted from the Twin Falls volcanic center of the SRP and a tuff from the Monterey Formation at Naples Beach in Santa Barbara, California. The second tuff (PRM-BC-2) correlates with another tuff in the Naples Beach section and likely erupted from the Picabo volcanic center of the SRP between 9.3 and 8.6 Ma. These correlations are consistent with an upper Mohnian age and the relatively high iron (3.13 and 2.58 wt% Fe₂O₃, respectively) and rare earth element concentrations are consistent with Yellowstone hotspot eruptions. Our results indicate that the air-fall distribution of the Tuff of McMullen Creek super eruption was at least 1,270 km from the SRP based on Miocene tectonic reconstructions of the Transverse Ranges. Ultimately, we correlate 20 ash beds at Balcom Canyon from the USGS database, which establishes a 9.3 to 7 Ma age for the section and demonstrates the temporal equivalence of the upper Modelo and upper Monterey Formations in the Transverse Ranges.