Uranium-series age of the Eel Point terrace, San Clemente Island, California

Uranium-series analysis of the coral Allopora californica Verrill from the 2nd, 32-m Eel Point terrace on San Clemente Island, California, has yielded an age of 127,000 k 7,000 yr. The Eel Point terrace is thus correlative with numerous terrace localities on the southern California mainland, with coral reefs on Barbados and New Guinea dated about 120,000 yr, and with substage 5e of the marine oxygen-isotope record. A tectonic uplift rate of about 0.20 m/1,000 yr has been calculated assuming a sea level slightly higher than the present one at the time of terrace formation. Extrapolation of this uplift rate allows age estimates to be made for other terraces on the island. INTRODUCTION acid racemization (Wehmiller and others, derived by kinetic models must be caliEmergent marine terraces are common 1977; Wehmiller and Belknap, 1978; brated, preferably by radiometric methods. along the coast of California and have been Kennedy, 1978; Lajoie and others, 1979; Uranium-series dating of mollusks has studied for many years. Age assignments Muhs, 1979a, 1980; Karrow and Bada, been attempted but has yielded discourfor terraces are difficult because of the lack 1980) and uranium-series dating. However, aging results (Kaufmann and others, 1971); of datable material. The two techniques because amino acid racemization is a mollusks d o not always act as closed systhat have been used most often are amino temperature-dependent reaction, ages tems with respect t o the migration of uranium isotopes and their long-lived Figure 1. Location map of San Clemente Island and Eel Point fossil locality. Pac i f i c daughter products. Unrecrystallized fossil corals have proven to be the most reliable sample type for dating by the U-series method (Ku, 1976); however, corals are rare in California terrace deposits. Previously, U-series dates on corals from California have been obtained only for terraces near San Diego (Ku and Kern, 1974), Cayucos (Veeh and Valentine, 1967), and San Nicholas Island (Valentine and Veeh, 1969). With the exception of a >200,000-yr date from the 9th terrace on San Nicholas Island, these coral dates range from 100,000 to 140,000 yr B.P. Fragments of the hydrocoralline coral Allopora californica Verrill, 1866, were collected from the 2nd, 32-m Eel Point terrace, on the west coast of San Clemente Island (Fig. 1). This terrace is the broadest and topographically best expressed geomorphic surface on the island. We present GEOLOGY. v 10, p 23-26, JANUARY 1982 23 TABLE 1 . ISOTOPIC A N D MINERALOGIC DATA FOR CORAL FRAGMENTS FROM E E L POINT, SAN CLEMENTE ISLAND, CALIFORNIA