GEOLOGIC MAP OF THE MORRO BAY SOUTH 7.5' QUADRANGLE

The California Geological Survey (CGS) has released a preliminary geologic map (1:24,000-scale) of the Morro Bay South 7.5’ Quadrangle. This map is the first to be completed in a multi-year project to prepare a seamless onshore/offshore geologic map of the of the San Luis Obispo 30 x 60-minute quadrangle in cooperation with the USGS and others. CGS will use this map and subsequent maps to prepare state-mandated Seismic Hazard Maps showing zones of required investigation for liquefaction and earthquake-induced landslides. These maps will also be used to improve estimates of earthquake ground shaking to be integrated into California’s building codes.

The Morro Bay South Quadrangle includes two structural blocks juxtaposed by the northwest-trending Los Osos Fault. On the southwest side of the fault, the San Luis Range is uplifting as a rigid block with a flight of up to twelve emergent marine terraces along the coast. Dating of these terraces by others indicates an uplift rate of 0.23 m/kyr. On the northeast side of the fault, the Cambria block is tilting southwest, with a deep basin south of Morro Bay and uplifted Quaternary fluvial deposits along Chorro Creek to the north. The geomorphic expression of the Los Osos Fault in the map area is weak, consisting of discontinuous tonal and topographic lineaments. The fault is poorly expressed in recent aeromagnetic and gravity maps prepared by the USGS. This suggests that the west end Los Osos Fault has a low rate of late Quaternary activity.

The San Luis Range is underlain by the Miocene Monterey and Pismo Formations. These rocks were folded in the late Miocene and Pliocene to form the Pismo syncline. The Pismo syncline extends through the southern part of the map area and continues offshore, where it is prominently expressed in high resolution bathymetric images acquired by CSU Monterey Bay. The Edna Fault is on the northeast limb of the Pismo syncline and has a complex history of displacement. It may have originated as an extensional fault during opening of the Pismo basin and later became a reverse fault during contraction of the Pismo syncline. Basement rocks of the Franciscan Complex are exposed in the Cambria block north of the Los Osos Fault. The Franciscan rocks are intruded by Oligocene dacite which forms distinctive peaks locally known as “The Morros” between Morro Bay and San Luis Obispo.