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.
Wheeler Ridge, located west of I-5 where it diverges from CA 99, is a splendid example of an actively growing anticline. The fold is so recent that unconsolidated sand and gravel sediments are involved. It is an asymmetric anticline, steep on the north side, with a thrust fault along its northern margin. Several topographic saddles or wind gaps notch the crest of the fold. They were cut across the growing anticline by a stream that flowed north from the San Emigdio Range south of the fold. Eventually the fold grew eastward, like a mouse under a rug, faster than the stream could erode, and so the stream was forced to turn aside and drain progressively around the east end of the growing fold, leaving the older gaps high and dry. Refer to Sharp and Glazner (1993) for additional information.
|View south of Wheeler Ridge anticline, which plunges from right to left. Four large aqueduct pipes occupy the large wind gap at the west and highest part of the anticline.|
1952 Kern County earthquake
One of California’s largest historic earthquakes (M7.5) happened along the southeast edge of the San Joaquin Valley on July 21, 1952. Considerable damage was done to the town of Tehachapi located on the hanging wall block of the fault. It was the largest earthquake to strike California since the 1906 San Francisco earthquake and fire, and the first in California after researchers installed the first generation of seismographs in southern California. Known as the Kern County, or Arvin-Tehachapi earthquake, it occurred on the White Wolf fault, a previously mapped, but undistinguished fault that lies along the south edge of the San Joaquin Valley against the Tehachapi Mountains. Because little surface rupture occurred, it was the seismographic record that produced the most information about the fault, its earthquake, and aftershocks. It turned out that the displacement was reverse, where Bear Mountain in the Tehachapi Mountains was thrust upward about 5 feet and leftward about 4 feet, relative to the floor of the San Joaquin Valley or, more likely, the San Joaquin Valley subsided about 5 feet relative to the Tehachapi Mountains. Over the last 2 million years or so, the vertical displacement along the White Wolf fault has been about 2 to 3 miles; the horizontal separation has not been determined.