Amino Acid Dating of Quaternary Marine Terraces, Bahia Asuncion, Baja California Sur, Mexico

Abstract

In the area of Bahia Asuncion, on the Pacific coast of Baja California peninsula, amino acid racemization dating' has been used to estimate ages of mollusks from Quaternary marine terraces. Eighteen molluscan samples (of the genera Tivela; Saxidomus; and Chione) from ten localities have been analyzed. The high mean annual temperature for the region (greater than 20 C) has resulted in extensive racemization of samples from what are considered to be late Middle and Late Pleistocene terrace localities. Racemization of most amino acids is effectively complete by about 300,000 years. However, two amino acids, leucine and valine, demonstrate enough resolving power to be used to delineate different age groups among the terrace sites. Where these apparent groups are testable with stratigraphic or geomorphic evidence, they are generally consistent with the available geologic control. The ages estimated for the three aminostratigraphic groups recognized in this study are approximately 120,000, 200,000 and 300,000 years.