Posted 22 April 2016 by A. G. Sylvester ©
Southern San Joaquin Basin
The southern San Joaquin basin is a prolific oil-producing area that has had a complex tectonic and depositional history. It originated as the southern part of a large forearc basin in late Mesozoic time, was affected by both shortening and extensional tectonics in Paleogene time, and underwent a final episode of subsidence and infilling during Neogene time under the influence of strike-slip deformation associated with the North American-Pacific plate boundary. During Neogene time, thick diatomaceous oil-prone source rocks of upper Miocene Monterey Formation were deposited in the southern part of the basin. They underwent subsidence and diagenesis, and generated large volumes of hydrocarbons. Numerous and thick sandstone bodies are interbedded with the diatomaceous strata and form the main reservoirs. Shelfal clastic units surround the basin on the east, south, southwest, and north. Large submarine canyons were cut across the shelves, probably during low sea level periods, and funneled sand basinward, where thick, sand-rich, submarine fans accumulated.
During Pliocene time, shallow-marine and intertidal conditions prevailed throughout the basin, because it was gradually cut off from the Pacific Ocean. During Pleistocene time, thick nonmarine strata were deposited in the basin, dominated by fluvial, lacustrine, alluvial-fan, and fan-delta depositional systems of the Tulare Formation and related units. The basin continues to undergo subsidence in its axial portions, concurrently with uplift along the western, southern, and eastern margins.
At least 40,000 feet of marine and terrestrial sedimentary rocks in the Maricopa sub basin are visible only in drill cuttings from the hundreds of boreholes that have punched the basin in the relentless search for oil and gas. The many pumpjacks along the route attest to the number and success of the drill holes. The drill cores may be studied at the California Well Sample Repository in Bakersfield.
The Maricopa sub basin lies upon metaigneous rocks that represent subsurface equivalents of ensimatic and ultramafic rocks that are exposed along the west side of the Sierra Nevada batholith and probably represent the floor of the batholith. The floor of the shallower Tejon embayment, by contrast, is crystalline granite, diorite or gneissic rock regarded as subsurface projections of exposed meta-tonalite and gneiss in the Tehachapi Mountains.
The oldest sedimentary rocks in the Maricopa sub basin are mid-Eocene shallow marine sandstone, siltstone, and shale beds that are overlain by Oligocene and Miocene volcanic rocks, coarse sandstone, and various conglomerates that interfinger with well-sorted, fine-medium grained marine sandstone and shale. The lower Miocene volcanic rocks have been dated at 22.7 million years and include a thick stacked sequence about 2,000 feet of basalt and dacite lava flows, lapilli and lithic tuff, volcaniclastic units, rhyodacite breccia, and dikes. The volcanic rocks are overlain by a very thick sequence of various shallow and deep water marine facies of sandstone, shale, and Monterey Shale of mid-Miocene age. Pliocene and Pleistocene deposits are represented by coarse alluvial fan and fluvial sediments, which interfinger with shallow marine shelf, brackish-water, and lacustrine deposits in the center of the basin. These basin center deposits indicate a gradual shoaling of the basin. The entire sedimentary succession contains major and minor unconformities, lateral and vertical facies changes, and is cut by various major and minor faults, all of which attest to the complex history of basin subsidence, deposition, and deformation of a huge pile of rocks that lie “under the rug” of the valley floor.
From Bakersfield to General Beale Road (Exit 127), the flatness of CA 58 belies the complex geology beneath Quaternary fluvial gravel and sand. North of the highway are dissected hills consisting of the Kern River Gravel, a non-marine, clastic, loosely consolidated, west-dipping alluvial fan sequence deposited not by the Kern River where it debouches into the valley, but instead by the now abandoned Caliente River (Saleeby et al, 2013). The deposits contain sand, pebble-sized gravel, rounded to sub-rounded cobbles and boulders, consisting of various igneous and metamorphic rocks, all derived from the core and west flank of the Sierra Nevada.
Tehachapi Mountains
Basinal marine strata interfinger with alluvial fan debris shed from the adjacent Tehachapi Mountains. The latest of these fans lies upon the valley floor close to the mountains. Some of them are dissected and are expressed by low hills before steep gradient of the highway begins.
The Bealville Fanglomerate is one of the older alluvial fan deposits in the Bakersfield area and provides the earliest evidence of strong uplift of the area east or southeast of the south end of the present San Joaquin Valley. Its age is regarded as late Oligocene and early Miocene based on stratigraphic relations. The formation consists of a light-gray, loosely consolidated mass of unsorted granitic detritus, with semi-rounded boulders of quartz diorite up to three feet in diameter. It nonconformably overlies granitic and meta-tonalitic basement and is more than 7,000 feet thick at Bealville. Most of the roadcuts farther up the grade from Keene (Exit 139) to Tehachapi expose granitic rocks – rarely foliated hornblende-biotite quartz diorite with hornblendite and gabbro in small irregular and linear masses. In the 3-5 miles before Tehachapi, mica schist and marble lenses are in the quartz diorite.
CA 58 crosses the Edison fault, which separates quartz diorite and Bealville Fanglomerate about two miles east of General Beale Road (Exit 127). It also crosses the White Wolf fault, source of the 1952 Arvin-Tehachapi earthquake (M 7.7); the fault juxtaposes diorite against diorite near the Bealville turnoff. The Tehachapi Mountains are not only a wall between the south end of the Great Valley and the Mojave Desert, but they also form a lithologic and structural link from the Sierra Nevada to the Transverse Ranges. They are also an impediment to highway and rail traffic. Railroad engineers solved their problem by building the Tehachapi Loop.
Tehachapi Loop
CA 58 and the railroad line climb 28 miles out of San Joaquin Valley over Tehachapi Pass to connect the valley with the Mojave Desert. Look for trains passing through tunnels on the south side of the highway. In just two years between 1874 and 1876, up to 3,000 Chinese laborers cut the route through solid and decomposed granite using picks, shovels, horse drawn carts, and blasting powder.
About halfway upgrade to the pass, the world famous Tehachapi Loop spirals 0.73 miles around one of the granite hills. The track climbs a steady 2 percent grade within the loop and passes over itself, thereby lessening the grade. The loop gains 77 feet in elevation, and thus a train more than 4,000 feet long thus passes over itself going around the loop. The average grade is a steep 2.2 percent. It is such a remarkable engineering feat that in 1998 the Loop was named a National Historic Civil Engineering Landmark and is now California Historical Landmark #508.
References cited:
Dibblee, T.W., Jr., and Chesterman, C.W., 1953, Geology of the Breckenridge Mountain quadrangle, California: California Division of Mines and Geology Bulletin, no. 168, 56 p., (incl. geologic map, scale 1:62,500).
Goodman, E.D., and P.E. Malin, 1992. Evolution of the southern San Joaquin basin and mid-Tertiary “transitional” tectonics, central California. Tectonics 11, 478-498.
Saleeby, J., Z. Saleeby, and F. Souza, 2013, From deep to modern time along the western Sierra Nevada foothills of California, San Joaquin to Kern River drainages. Geological Society of America Field Guides 32, 37-62. doi: 10.1130/2013.0032(03)
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