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.
Description:
Includes bibliographical references (pages 78-85)
California State University, Northridge. Department of Geological Sciences.