HOLOCENE STRATIGRAPHY AND TIMING OF SEA LEVEL ON-LAP, HUNTINGTON BEACH, CALIFORNIA

Recent subsurface exploration was conducted at a near-shoreline site on the coastal plane, mid-way between Huntington Mesa on the north and Newport Mesa on the south, in Huntington Beach, California. The site stratigraphy consists of nearly 18 m of estuarine and littoral sediments overlying fluvial sediments extending to greater than 27 m. Nine fine-grained marker beds were correlated between 6 boreholes and 21 cone penetration test locations. Radiocarbon dating of organic matter in the fine-grained deposits and shells in the littoral sands reveal the section spans more than 12,000 ybp. On-lap of rising Holocene sea level occurred at the site approximately 8,000 ybp. Prior to on-lap, the coastal plane aggraded at a rate of approximately 1 m/1,000 yr. Three dates between 7,090 ybp and 8,040 ybp (representing 9 m of section) plot directly on the sea level curve, indicating coastal aggradation kept pace with sea level rise for about 900 years. Thereafter, the rate of sea level rise out-stripped deposition, and the shoreline lay to the east of the site. The coastal plane has aggraded at a rate of approximately 1.3 m/1000 yr to the present time. Exploratory boreholes and test sites were arrayed in three northeast-southwest transects oriented transverse to the trend of a postulated south branch of the Newport-Inglewood fault zone. No evidence of vertical separation of the fine grained marker beds was discernible, suggesting the section has not been affected by faulting.