Abstract: Magnetic Stratigraphy of Plio-Pleistocene Sediments of the Los Angeles Basin, California

HILLHOUSE, JOHN W.

The age of Pliocene and Pleistocene deposits in the Los Angeles Basin is critical in measuring rates of tectonic deformation and ultimately in assessing seismic risk posed by buried thrusts. The Pliocene and younger stratigraphy of the Los Angeles Basin is relatively well known from well logs and seismic imaging; contours of major surfaces, such as the Pico-Repetto contact and the San Pedro-Pico contact, are mapped throughout the basin. Age control of these deposits, however, is not well known; ages have been assigned mainly on the basis of biostratigraphic correlations that are thought to be time transgressive. Magnetic stratigraphy offers improved age control in the basin from subsurface sediment cores. I have obtained core samples for paleomagnetic polarity determinations from 6 locations from borings as deep as 915 feet. The first magnetic timeline in the search is the Matuyama-Brunhes transition (780,000 years ago), which occurs 90 feet below the surface at La Brea and is deeper than 915 feet, the maximum depth cored, at the intersection of La Cienega Blvd. and 1-10. Bore holes as deep as 850 feet at Gardena and Lakewood yielded normal polarity only, indicating that the Matuyama/Brunhes boundary lies at greater depth. More bore holes are scheduled to be drilled in the course of earthquake-risk and groundwater studies, providing opportunities to expand this preliminary effort to identify and map paleomagnetic timelines in the Los Angeles Basin.

Search and Discovery Article #90945©1997 AAPG Pacific Section Meeting, Bakersfield, California