MIOCENE VOLCANO EXPOSED IN CROSS SECTION, SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

The lower Miocene (Ar-Ar age: 16.3-17.1 Ma) Conejo Volcanics, exposed in and around the Santa Monica Mountains of southern California, offer a unique opportunity to study an exposed cross section of the interior of a volcanic mountain and its surrounding volcanic field. The volcanics consist mostly of basalt and basaltic andesite flows and lahar deposits and extend east-west for 80 km from Griffith Park to the Oxnard Plain and north-south 35 km from Big Mountain to the Pacific Ocean. An isopach map of the volcanics shows that the volcanic pile consisted of three separate shield-shaped cones, the highest near the western end of the Santa Monica Mountains ranging to >9,000 m in thickness. Pillow lavas and oyster shells indicate a submarine origin for the base of the volcanic pile, whereas spatter cones of bombs and lahar deposits containing petrified wood indicate subaerial accumulation by the end of the eruptive cycle. Fossil wood species in the lahars indicate an above-sea-level elevation of at least 1,500 m. After the volcanic cycle ended, the volcanic mountain was tilted northward during post-Miocene deformation of the Santa Monica Mountains and eroded so as to reveal its interior from the base against the underlying lower Miocene Topanga Canyon Formation to its top against the overlying middle Miocene Calabasas Formation. Interior details thus revealed include overlap of volcanic flows onto the subjacent sedimentary rocks during growth of the volcanic cones, a filled lava tube, breakup of the cones into large blocks during intrusion of feeder dikes and sills, and intrusion of a volcanic neck through the underlying Topanga Canyon Formation and into and through the volcanic rocks.