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Description
The Mission Valley Formation is located in western San Diego County, California. It is composed of mixed marine and nonmarine facies, with the nonmarine facies predominantly to the east. The nonmarine facies contain caliche. The caliche-bearing sandstones and mudstones are mainly feldspathic volcanic litharenites. The mineral fraction of the sandstones probably originated in the Southern California Batholith. The volcanic rock fragments most likely came from the Santiago Peak Volcanics or from Sonora, Mexico. Many of the quartz grains show a peculiar, unexplained fracturing. The caliche in the Mission Valley Formation was deposited by pedologic processes and may be divided into five types based mainly on morphology. Type I caliche is topographically controlled and drapes over the modern topography. Its age is post-Eocene. Type II caliche is stratigraphically controlled and is Eocene in age. Type III caliche is veined and results from redeposition of types I and II. Type IV caliche is nodular and found only in the Penasquitos area. It is of Eocene age. Type V caliche occurs as calcite cement, and it may or may not be of Eocene age. The multilayering of type II caliche suggests an aggrading environment. Land vertebrate fossils are found within the Mission Valley Formation mudstones associated with the caliches. The fossils suggest a more humid climate in the Eocene. To account for semi-arid caliches and fossils from a humid environment, a savannah-type climate apparently existed during deposition of the Mission Valley Formation. During the wet season, material was washed into a river valley or flood plain. With the coming of the dry season, caliche was formed by capillary action in the soils. This cycle was repeated, building the multilayered caliche.
