T60 Earth Surface Processes in Tectonically Active and Unstable Regions II

Sunday, 22 September 2024
1:30 PM - 5:30 PM
210B

FIELD AND PHOTOGRAPHIC INVESTIGATIONS: EXPLORING DISTINCTIVE GEOLOGICAL STRUCTURES AND VOLCANIC PHENOMENA IN THE WESTERN SANTA MONICA MOUNTAINS

Paper number: T60-12
Program: Technical Sessions
Time: 4:20 PM – 4:35 PM

Authors

Abstract

Nestled within the Western Santa Monica Mountains of Southern California, Conejo Mountain stands as one of 16 extinct volcanic centers that were examined in this study. Our field investigations confirmed the rugged present day terrain, along with multiple dacite and volcanic flow breccia outcrops, some of which contain angular clasts measuring over 10 inches in diameter. Positioned on the eastern flank of the Oxnard Plain, Conejo Mountain reveals a fascinating geologic narrative. Adjacent to the mountain's western face lies a northwest trending fault zone, where subparallel fractures serve as a conduit for surface hydrocarbon seep migration upward through and into Camarillo Springs golf course. Verified as a 1920's oilfield, the fault zone with field lineaments, seismic activity, and ground shaking may have caused oil seeps, rockfalls and rock slides at the site.

The area also boasts three Conejo Volcanic plugs: Tarantula Hill, Strawberry Hill, and University Hill. These plugs, with their distinctive funnel or elliptical shapes, are erosion resistant due to their finely crystalline, volcanic nature, and composed of dacite or porphyritic andesite. Formed from magma that solidified at shallow crustal depths, these plugs were once part of volcanoes, possibly reaching the level of ancient volcanic vents.

Venturing further west, one encounters the distinctive horizontal columnar jointing at Conejo Grade, where it flanks both sides of the 101 Freeway close to Camarillo Springs. These structures formed as magma cooled and crystallized as an andesitic dike, offering a glimpse into some of the processes that shaped the Earth's crust. Their horizontal orientation signifies the gradual cooling and shrinkage of magma as well as the sub-vertical dike walls.

The area was also shaped by magmatic intrusions, in an ancient underwater environment, where pillow basalts were extruded onto the seafloor. Some basalt pillows exceed 3 feet in diameter and exhibit baked zones along Kanan Road and Yerba Buena Road.

Between Point Mugu Rock and Malibu lies a diverse array of lava flows, dikes, sills, stocks, fissures, and laharic flows containing volcanic bombs. These features are and evidence of millennia of dynamic forces. In summary, the western Santa Monica Mountains tell a tale of ancient events, of remnants of volcanic eruptions and faults zones in a landscape rich in history and natural beauty.

Geological Society of America Abstracts with Programs. Vol. 56, No. 5

doi: 10.1130/abs/2024AM-400294

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