Geology of the inner basin margin, Dana Point to San Onofre, California

Abstract

Acoustic subbottom profiles, in conjunction with onshore geological data and limited bottom samples were analyzed to determine the geology and Quaternary evolution of the inner basin margin between Dana Point and San Onofre, California. Structural features are characterized by the typical northwest trends of the Peninsular Range Province and the Southern California Continental Borderland. The major structural element of the study area is the offshore Newport-Inglewood fault zone. Activity has not been continuous over the length of the offshore Newport-Inglewood fault zone. Within the study area, sea floor scarps and offset Holocene sediments are evidence of Holocene activity south of San Mateo Point. Subbottom profiles, dart cores and vibro-cores were correlated with the onshore data to map the offshore geology. Exposed units range in age from the mid-Miocene San Onofre Breccia to Holocene sediments. The San Onofre Breccia unconformably overlies the basement unit which is Catalina Schist. The deep water Monterey and Capistrano Formations overlie the San Onofre Breccia. During the last low stand of sea level (20,000 years b.p.) a bedrock erosional shelf surface was cut. Three units of late Pleistocene age unconformably overlie this erosional surface. At least two buried offshore terraces, which are incised into late Pleistocene sediments, have been mapped within the study area. In addition, nine coastal marine terraces have been identified. During periods of sea level fluctuations, uplift rates of 6 cm/1000 years to the south and 24 cm/1000 years to the north caused a regional tilt to the north and formed the marine terraces. Sea cliffs along the modern coast suggest a continuation of the process. The modern shelf is blanketed with latest Pleistocene/Holocene sediments. Sediment volumes were calculated along the shelf from San Pedro to the Mexican border. Anomalously high volumes of sediments are present within the study area. The offshore terraces cut into the soft Pleistocene units may have acted as dams, trapping large volumes of marine sediments during the last transgression.