--> Abstract: Fossil Wood in a 13.5 Ma Tuff Breccia of the Conejo Volcanics, Santa Monica Mountains, California, by C. J. Stadum; #90935 (1998).

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Abstract: Fossil Wood in a 13.5 Ma Tuff Breccia of the Conejo Volcanics, Santa Monica Mountains, California

STADUM, C. J., RMW Paleo Associates, Inc., Mission Viejo, CA

The resistant ridges of the Santa Monica Mountains are composed of the Conejo Volcanics, a formation that commenced as submarine basalt flows 16 Ma and concluded in a series of explosive lahars 13.5 Ma. Construction activity in the tuff breccia from the most recent lahar event exposed two fossil logs and 130 fragments of silicified wood in the City of Westlake Village. Preservation of the wood varies from splintered surfaces and detailed cell features to amorphous clayey limb casts. Ponderosa pine, Douglas fir, incense cedar, and specimens from the dicotyledonous families Fagaceae (evergreen and deciduous oaks), Rosaceae (mountain mahogany), and Rhamnaceae (mountain lilac) have been identified by thin section analysis. Notably missing from the Conejo assemblage are palms, avocados, and sycamores, common fossil wood in other Los Angeles Basin Miocene formations.

The Conejo assemblage represents a mixed conifer/hardwood forest similar to present-day transitional forests. These forests live in a low montane zone from 700 to 1800 m. During the warm middle Miocene, this zone was ~600 m higher. Temperature decline over the last 2 million years has caused this zone to move gradually down slope. This indicates that the elevation of the Conejo volcanic highlands upon which the trees grew was >1300 m. The Conejo fossil wood represents a low montane geoflora previously undescribed from the middle Miocene formations of the Los Angeles Basin region and is the first record of fossil wood in a tuff breccia from the Santa Monica Mountains.

AAPG Search and Discovery Article #90935©1998 AAPG Pacific Section Meeting, Ventura, California