Geology of the Ventura Hills, California

Geo-hikers, I'll be giving a presentation on the Geology of the Ventura Hills and Environs, California, April 24, 2025 @ 7:00 PM. The event is free and at 205 N Garden St. Ventura, CA, 93001-2532. If you intend to go, please register via The Ventura Land Trust FB page: https://www.facebook.com/events/9487724331344128/

Geology of the Ventura Hills and Environs (VE), California
Thomas L. Davis, Geologist¹ – April 24, 2025
This geologic tour highlights key geologic features of Ventura and its surrounding areas (VE) through photos, maps, cross-sections, and lecture. The region's steep and rugged Ventura Hills, the Santa Ynez–Topatopa Range to the north, and frequent earthquakes all reflect the area's active tectonics. VE lies near the Pacific–North American (PNA) plate boundary—currently defined by the San Andreas Fault—and its geologic evolution has played a critical role in shaping the landscape and subsurface geology of VE. The rock record of VE chronicles the evolution of the PNA boundary over the past ~160 million years (Ma)². Before 25–30 Ma, the boundary was convergent, as the Farallon oceanic plate subducted eastward beneath the western margin of North America. Northwest of VE, along the Santa Ynez Fault, fragments of subduction-related rocks (e.g., serpentinite and the Franciscan Complex) were scraped from the descending Farallon plate and accreted to North America between ~80–160 Ma. North of Ojai, outcrops of marine shale, sandstone, and conglomerate represent remnants of an extensive forearc basin that once stretched across Upper and Lower (Baja) California from ~80–40 Ma. This basin, situated between the Sierra Nevada–Peninsular Range magmatic arcs and the subduction zone to the west, was later disrupted by faulting related to the evolving San Andreas system. Around 28–25 Ma, the plate boundary transitioned from subduction to transform motion as the Rivera Triple Junction (involving the Farallon, Pacific, and North American plates) migrated southward. This tectonic shift marked the birth of the right-lateral San Andreas Fault and initiated the formation of crustal depressions like the Ventura Basin. Pale-orange and whitish shale exposures of the Monterey Formation, visible along Hwy 33 north of Casitas Springs, are deep-marine deposits that mark a period of maximum marine transgression.
VE lies within the Western Transverse Ranges fold-and-thrust belt, an east-west trending zone characterized by large-scale folds and reverse/thrust faults³. Over the past 3–4 Ma, southward migration of this belt has caused crustal convergence and deformation of the Ventura Basin, documented by tilted sedimentary layers and compressive earthquakes. This convergence has shortened the crust in a north–south direction and thickened it vertically through stacking of older over younger rocks, producing rapid uplift. These complex processes—including strain partitioning and mantle-lithosphere delamination—are best illustrated through figures. Geologic mapping and oil well records show the Ventura Hills are underlain by the massive Ventura Anticline⁴, an east-west trending fold flanked by the Canada Larga Syncline to the north and the thick Pliocene–Quaternary Ventura Basin to the south and east. The anticline's limbs dip steeply, and it plunges steeply eastward into the Ventura Trough (near Saticoy), where it terminates. Onshore, the Ventura Anticline traps the Ventura Avenue and San Miguelito oil fields. Offshore, the fold continues as the Dos Cuadras Trend. The Ventura Avenue Field alone has trapped over 3 billion barrels of oil (~1 billion produced, ~50 million in proven reserves, ~2 billion unproducible). Subsurface imaging reveals a far more complex picture than surface mapping suggests. Oil-bearing zones are segmented by numerous north- and south-dipping thrust faults, and some interpretations propose a detachment fault at ~7 km depth that may link northward to the San Cayetano Fault or southward to the offshore Oak Ridge Fault.
Along the southern limb of the Ventura Anticline, beneath the City of Ventura, the Ventura–Pitas Point Fault reaches the surface and is classified as an active surface rupture hazard under California’s Alquist-Priolo Earthquake Fault Zoning Act (1972) and the Seismic Hazards Mapping Act (1990). Hubbard et al. (2024) propose that the fault can generate large (Mw 7–8) earthquakes with several meters of displacement and potential tsunamis. In contrast, Yeats (1982a, b), drawing from deep oil well data, argued the fault is inactive, possibly nonexistent, and that subsurface evidence is lacking while surface data remains open to multiple interpretations. The VE coastline and the Northern Channel Islands preserve a record of global climate changes over the past several hundred thousand years, as seen in features shaped by sea-level fluctuations: marine terrace deposits, wave-cut platforms, and relict sea cliffs. West of the Ventura River outlet (north according to CalTrans signage), two prominent marine terraces are visible on coastal hillsides extending beyond Rincon Point. Highway 101 runs along the lower terrace, dated via radiocarbon to ~5,000 to ~1,800 years BP². Rockwell et al. (2016) further subdivided this terrace into four distinct levels, dated at ~6,700, ~4,400, ~2,100, and ~950 years BP—each interpreted as marking the uplift from a major earthquake. The upper terrace, towering over the highway between ~100–280 meters above sea level, has amino acid ratios suggesting an age of ~50,000 years BP and an uplift rate of 5–6 mm/year—comparable to rates in the frontal Himalayas.
¹ https://geologicmapsfoundation.org | https://thomasldavisgeologist.com
² Ma = millions of years ago; ka = thousands of years ago; BP (Before Present) = years before 1950 CE
³ A thrust fault places older rocks over younger ones on a gently dipping surface; a reverse fault has a steeper dip.
⁴ Anticlines are upward-arching folds with oldest rocks in the core; synclines are downward troughs with younger rocks in the center.