The Miocene Monterey Formation

The Los Angeles Basin is the most productive basin by area in the world – and the Miocene Monterey Formation, which is exposed along the coast and in the hills of the Palos Verdes Peninsula, is the prolific source rock for this basin.
This article appeared in Vol. 9, No. 6 - 2013


Neogene basins of California. Source: After Behl, 2012, used with permission Lower Monterey Formation ‘burnt shale’ visible on the cliff face at Lunada Bay – a pink to red-colored rock which shows where organic-rich rocks have spontaneously combusted and oxidized. They contain a mixture of minerals that formed in a high temperature, low pressure anhydrous environment. Some burn for many years and their fumaroles of steam can appear after major rainstorms, when water seeps far below the surface to the hot spots. The Monterey Formation is a bio-siliceous, very organic-rich deposit found in southern California. It was deposited between 17 and 5 million years ago during a time when tectonic forces were shifting, and localized subsidence during a time of high eustatic sea level, along with coastal upwelling, affected the area. The Monterey and its equivalents, the Puente and the Modelo Formations, are the primary source rocks for oil production in the Los Angeles Basin.

Thanks to a field trip offered at the recent AAPG convention in April of 2012 at Long Beach, California, participants were able to visit the Monterey in the tectonically active highlands surrounding the Los Angeles Basin, in particular along the coast on the Palos Verdes peninsula west of downtown Los Angeles, where a 600m thick section of the Monterey is exposed. Dr. Richard Behl of California State University at Long Beach, who has studied this region for many years, led the trip.

Complex Geological History

Phosphatic marlstone or mudstone/shale outcrop. Organic-rich, bituminous, calcareous phosphatic shales are interbedded with dark mudstone, Lunada Bay, Palos Verdes Peninsula, Los Angeles. Source: Anne Hargreaves The geological history of California is quite complex, but the following is a brief summary. 200 million years ago the western edge of North America was approximately at the current border of California and Nevada. Then, as the mid-Atlantic Ridge began to spread, North America began to move west colliding with the floor of the Pacific Ocean, which was subducted beneath it. Oceanic sediments and volcanic islands were too buoyant to sink, so they crumpled into the North American continent, eventually becoming California.

This happened in three main events. The first one, during the Mississippian, was the Antler orogeny which created the Shoo Fly metamorphic complex, now part of the eastern Sierra Nevada. The Shoo Fly complex became the ‘new’ west coast. Next, when the mid-Atlantic ridge began to separate in earnest at the end of the Triassic, the new coastal deposits being shed off the Shoo Fly were compressed and accreted onto the continent, becoming the Calveras complex, which is now a central range in the Sierra Nevada. At the same time, an old volcanic island arc was included in the accreted mix, becoming the Western Jurassic Terrane. The third important event occurred in early Cretaceous times, when the subduction zone jumped west by 60 km for an unknown reason, creating an inland sea over what was to become the Great Valley. Also at this time a chunk of the Sierra Nevada detached and moved west, becoming the Klamath Mountains of northern California. A new trench began to form offshore: the Franciscan Trench, which collected sediment during the Cretaceous until the Oligocene. At that time general uplift created the Coast Range on the newest west coast, and the broad expanse of the Great Valley, now an inland sea, continued to fill with sediments.

In the middle of the Miocene, about 17 million years ago, a lot of volcanic activity in Oregon resulted in massive flood basalt flows over that state and in California. One theory is that a huge meteorite hit south-east Oregon, allowing more than 100 cubic miles of lava easy access to the surface through the resulting fractures.

At this time, the San Andreas fault (SAF) came into its own. This strike-slip fault occurs at the juncture between the Pacific plate and the North American plate. Currently the Pacific plate moves northward at a rate of two inches per year and to date has carried California west of the fault northward at least 560 km.

Meanwhile the Basin and Range areas east of the SAF began to form due to crustal tension and stretching. This province consists of isolated mountain ranges separated by desert plains. It is estimated that this area has widened and thinned, and is currently twice as wide as it was before the great event which initiated this action around 17 million years ago.

Finally, the Coast Ranges began to uplift and deposition of the Monterey Formation commenced.

Wide Range of Sediments

Schist and phosphatic breccia rubble from the lower Altamira turbidite deposit (Lower Monterey Formation). Source: Anne Hargreaves Relationshipws between silica phases, temperature and sediment composition. Source: After Behl, 2012, used with permission The Los Angeles Basin is one of more than 20 Neogene basins in California that were created due to action on the boundary of the North American and Pacific plates. The basins are part of the California Continental Borderland where sedimentary basins alternate with ridges to the west of the SAF; one of which is the Palos Verdes Peninsula; another being Catalina Island. The area that is the city of Los Angeles today is one of the Borderland Basins and was formerly under water, eventually in-filled with sediment deposits shed from the surrounding mountain ranges.

The section of the Monterey Formation found on Palos Verdes has been transported northward approximately 20–30 km from where deposition first occurred. The lower Altamira formation was deposited directly on the early Mesozoic Catalina Schist basement. In fact, the distinctive blue schist can be seen as rip up clasts in these basal breccia and/or conglomerate sediments. Overall it thins to the north and east, and shows great lateral variation in thickness, degree of deformation and stratigraphic continuity.

The Monterey deposits include carbonaceous, calcareous and phosphatic mudrock, dolostone, limestone and marlstone in addition to siliceous diatomite, porcelanite and chert. It was the massive amount of diatomite created from many millions of silica-rich diatoms that provided the oil that supplied the Los Angeles Basin. In fact, the Monterey Formation can have an organic content as high as 24%. Due to the high sulfur content, it generates hydrocarbons at an early stage and without being as deeply buried as one would expect.

Silica Diagensis Important

Anne Hargreaves has a B.Sc. in Geology from the University of Calgary. She has worked for over 20 years for several companies working with geological data and collections. Currently she is Vice-President of Canadian Stratigraphic Services (2000) Ltd, which is better known by the trade names of Canstrat and Amstrat, and which supplies well log descriptions from Canada and the US to the oil and gas industry. Weathered piece of siliceous shale sandwiching a thin chert layer in the Monterey Formation – this is a perfect example of mechanical stratigraphy which is so important in exploiting unconventional reservoirs. The chert layer is highly jointed and fractured, but the shale above and below is not, even though they were all exposed to the same stress field. Undoubtedly weathering has deepened the joints perpendicular to the bedding plane. Concentrically fractured chert nodules or spheroids indicate early and shallow silica diagenesis. Source: Jane Whaley Detailed study of the Monterey Formation has allowed for extensive insight into silica diagenesis and chert and porcelanite formation, as it is possible to see all stages of burial diagenesis and all silica phases and the significance of these on the petroleum geology of the basin.

Silica phases observed include: a) biogenetic opal-A which is a hydrous silica found in the shells of diatoms and radiolarians, many of which were deposited in the Monterey; b) metastable opal-CT which is hydrous silica which forms with burial, increased temperature or the passage of time as an alteration product from opal A through dissolution and re-precipitation; and finally c) the end result of stable diagenetic quartz, which is found as fibrous chalcedony or cryptocrystalline or microcrystalline quartz or chert. Temperature and time contribute to these three phases, but the amount of clay, organic matter and calcium carbonate also affect the amount of silica diagenesis. The presence of clay and organic matter slows the opal-A change to opal-CT, but the presence of calcium carbonate speeds up the formation of opal-CT and also possibly quartz formation.

Although both diagenetic siliceous rocks, chert and porcelanite show the importance that additional material such as clay make to the final rock. Chert is a fairly pure siliceous rock which is dense and hard, with a smooth conchoidal fracture and waxy luster, and normally is composed of 90 to 95% diagenetic silica. Porcelanite, on the other hand is only 50–85% diagenetic silica, with the rest being clay content and/or porosity. This makes it less dense than chert, with a blocky to splintery fracture and a matte surface resembling unglazed porcelain – hence the name. The porosity can be as much as 15–25% and often it is layered with dark shale.


Valmonte diatomite member of the Monterey Formation – white to light grey laminated diatomite and diatomaceous shale with some greybrown Opal-CT chert beds and nodules in Del Cerro park, Rancho Palos Verdes, LA. Source: Anne Hargreaves Alt, David and Hyndman, D.W., Roadside Geology of Northern and Central California, Mountain Press Publishing, Missoula, MT, 2000.

Behl, Richard J., Guidebook to Miocene Monterey Formation of the Los Angeles Basin, CSULB, April 21, 2012.

Clarke, D. The Road to Bakersfield. In: Heavy Oil Production and Outcrops on the East Flank of the San Joaquin Valley; AAPG 2012 Annual Convention Field Trip 15 Guidebook; Energy Minerals Division and the San Joaquin Geological Society, 2012.

Sharp, R.P. and Glazner, A. F., Geology Underfoot in Southern California, Mountain Press Publishing, Missoula, MT, 1993.

Acknowledgement: This article was previously published in the CSPG magazine Reservoir, Vol. 39, No. 9, and is reprinted with permission.


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