PRE-AND POST-MODELO FOLDING OF SEDIMENTARY ROCKS IN THE EASTERN SANTA MONICA MOUNTAINS, SOUTHERN CALIFORNIA

Approximately 45 years ago, a regional study in the Santa Monica Mountains was started by mapping and evaluating multiple fault splays and folded sedimentary beds adjacent to the Benedict Canyon Fault zone. Specifically, the sites are located in the Laurel Canyon area, south of Ventura Boulevard.

The complex geologic assessment required the revising of several working hypothesis for the geologic outcrops observed along the Benedict Canyon Fault zone. The fault zone and its splays are responsible for the overturning and folding of the sedimentary rocks that occurred at several sites in Studio City. Three of the smaller splay faults are terminated by a Cretaceous-age quartz diorite. The fourth splay fault extends around the quartz diorite intrusion, and is responsible for much of the overturning and folding of the sedimentary rocks directly related to numerous active landslides affecting multiple sites within the eastern Santa Monica Mountains.

The acceptable geologic model for this study area consisted of north dipping planes on the north flank of the Santa Monica Mountains anticline. However, within this study area bedding planes either dipped to the south at steep angles or it was observed that there were multiple folds. Over time the study area was extended to include both pre Modelo Formation and some younger sediments and older metamorphic rocks as far as to the east of the San Diego Freeway. Numerous geologic maps were also compiling of these different sites by the lead author at a scale of 1” = 100’.

Thus, the study area became centered along the folding long the left-lateral Benedict Canyon fault zone. New geologic data was generated and collected within the general study area during site grading operations of either new residential developments, or the numerous exposed rock outcrops in the study area.

The additional mapping of the complex folding of the sedimentary rocks during grading operations provided answered to the original geologic questions about the folding of these rocks. This area of intense folding is in both the pre and post Modelo formation rocks, that are located to the north of the maximum bend of the Benedict Canyon Fault, where the bend of the fault and the splinter faults takes place around a dense body of intrusive crystalline basement rocks made up of intrusive Cretaceous quartz diorite.