Geology of San Diego County, California
San Diego County can be divided between three distinct geomorphic regions: the Coastal Plain region as exposed west of the Peninsular Ranges, the Peninsular Range region, and the Salton Trough region as exposed east of the Peninsular Ranges. This geomorphic division reflects a basic geologic difference between the three regions, with Mesozoic metavolcanic, metasedimentary, and plutonic rocks predominating in the Peninsular Ranges, and primarily Cenozoic sedimentary rocks predominating to the west and east of the central mountain range. The irregular contact between these geologic regions reflects the ancient topography of this area before it was buried by the thick sequence of Cretaceous and Tertiary sedimentary rocks deposited over the last 75 million years by ancient rivers and in ancient seas.
Together, these rocks and their contained biological record (i.e., fossils) document the geological and biological evolution of this part of western North America. They record a time when consumption of an ancient oceanic crustal plate created an archipelago of volcanic islands, and when this same plate subduction produced massive volumes of magma that later congealed in the crust to form plutonic rock.
Local rocks also record an early period when tectonic forces uplifted and erosion unroofed the deeply buried plutonic rocks to form a steep and rugged mountainous coastline, similar to that present today along the west coast of South America.
Also recorded are periods of higher rainfall and subtropical climates, which supported coastal rain forests with exotic faunas and floras; periods of drowned coastlines, great rivers, and relentless erosion; periods of extreme aridity and renewed volcanism; and periods of widespread faulting and crustal shear and the formation of new seaways.
These are just some of the geologic stories preserved in the rocks of San Diego County. Deciphering the geological and biological record is an ongoing process and each year brings new discoveries and new insights. In the sections that follow, more specific information is provided for the different regions of our area.
In the Coastal Plain region, resistant peaks composed of Mesozoic crystalline rocks (such as at Rock Mountain on the north side of Otay Valley, Black Mountain near Rancho Penasquitos, and Cowles Mountain near San Carlos) are actually "rooted" at depth to the buried Mesozoic crystalline rock terrain. These basement "highs" poke through the younger Cretaceous and Tertiary sedimentary cover and demonstrate the amount of topographic relief on the buried landscape of western San Diego County.
The Coastal Plain Region is underlain by a "layer cake" sequence of marine and non-marine sedimentary rock units that record portions of the last 140 million years of earth history. Over this period of time the relationship of land and sea has fluctuated drastically so that today we have ancient marine rocks preserved up to elevations around 900 feet above sea level and ancient river deposits as high as 1,200 feet.
Faulting related to the local La Nacion and Rose Canyon fault zones has broken up this "layer cake" sedimentary sequence into a number of distinct fault blocks in the southwestern part of the county. North of La Jolla the effects of faulting are not as great and the rock units here are relatively undeformed.
Excellent exposures of late Cretaceous-aged (72-76 million years old) marine sedimentary rocks occur in the sea cliffs along the west side of the Point Loma Peninsula and in La Jolla from Bird Rock to La Jolla Shores. The sea cliffs north of Scripps Institution of Oceanography provide spectacular exposures of Eocene-aged (42-48 million years old) marine sedimentary rocks.
The Peninsular Ranges Region is underlain primarily by plutonic (i.e., granitic) rocks that formed from the cooling of molten magmas deep within the earth's crust. These magmas were generated during subduction of an oceanic crustal plate that was converging on the North American Plate between 140 and 90 million years ago.
Over this long period of time, extensive masses of granitic rocks accumulated at depth to form the Southern California Batholith. Intense heat associated with these plutonic magmas metamorphosed the ancient sedimentary rocks into which the plutons intruded. These metasediments are now preserved in the Peninsular Range Region as marbles, slates, schist, quartzites, and gneiss.
Good exposures of schist can be seen in the roadcuts along Banner Grade and exposures of marbles can be found in the Ranchita area. In Jacumba Valley there are younger, Miocene-age (16 million year old) volcanic rocks and flows. Even younger sedimentary rocks occur in isolated districts in various regions of the Peninsular Ranges Region, such as in Warner Valley.
Approximately the eastern one-third of San Diego County falls within the Salton Trough Region, also referred to as the Colorado Desert. The Salton Trough is the northern landward extension of the Gulf of California and is undergoing active deformation related to faulting along the San Jacinto and Elsinore fault zones. These fault zones are in turn related to the major tectonic feature in this part of the world -- the San Andreas Fault.
Much of the land surrounding the Salton Sea in the Imperial and Coachella valleys is below present sea level. This is the result of crustal thinning and subsidence caused by the same extensional tectonics that continue to form the Gulf of California today. As a result of this rifting and subsidence, the Salton Trough has been filled with sediments to a depth of up to 5 miles since the early Miocene, approximately 24 million years ago. The source of these sediments has been the local mountain ranges, as well as the ancestral and modern Colorado River.
Wonderful exposures of late Miocene-aged marine sedimentary rocks are found in the Coyote Mountains and Fish Creek Mountains. The Borrego Badlands in Anza Borrego Desert State Park provides extensive exposures of Pleistocene-aged (100-700 thousand years old) stream and playa lake deposits.