Espada Formation
Espada Formation | |
---|---|
Franciscan Formation[2] | |
Thickness | Up to 16,000 feet (4,900 meters) [3] |
Lithology | |
Primary | Shale, sandstone |
Location | |
Region | Coastal and interior southern California |
Country | United States |
Type section | |
Named by | Thomas Dibblee (1950)[4] |
The Espada Formation is a
Type locality and description
The type locality of the Espada Formation is in Hondo Canyon, near Point Arguello, as first described by Thomas Dibblee in his 1950 book on the geology of southwestern Santa Barbara County.[4] While the unit is 1,200 meters (3,900 feet) thick at the type locality, it is much thicker elsewhere; along the southern slope of the San Rafael Mountains it is exposed from its base to its top, where it is probably in conformable contact with the Jalama Formation, and the total thickness is around 5,300 meters (17,400 feet) – 5 kilometers (three miles) of uninterrupted sedimentary deposition that took place over 50 million years.[3][5]
The formation throughout its geographic range consists of layer upon layer of well-bedded
Deposition environment and geologic history
The Espada Formation represents a long period of geologic history – tens of millions of years – in which sediments were deposited in warm, quiet water in a basin that was gradually subsiding. The water was usually shallow, as indicated by the frequent sandstone beds (coarse sediment such as sand is deposited nearer shore unless carried far offshore by submarine landslides and other subsea mass movements).
The crustal block on which the Espada was deposited has been shifted northwestward along the plate boundaries and rotated approximately 90 degrees since the time of its deposition. Most of the movement took place recently, compared to the time of deposition – in the Miocene and Pliocene eras.[9] As the block shifted northward, the formation was also subject to folding, resulting in creation of structures such as the Mono syncline, which includes Camuesa Peak north of Gibraltar Reservoir, and the parallel Agua Caliente anticline.[3]
Paleontology
Relatively few fossils are found in the Espada, considering its extent and immense depth, but those that are found mostly indicate a Cretaceous age. Buchia piochii, found in the lowest part of the Espada, has been dated to the late Jurassic, which gives the low extreme for age of the unit; Coralliochana orcutti is found near the top, indicating late Cretaceous age and warm water, suggestive of tropical conditions. Other fossils found include
Notes
- ^ Thomas M. Dibblee, 1986. Geologic Map of the Carpinteria Quadrangle, Santa Barbara County, California. California Department of Conservation, Division of Mines and Geology. Dibblee Foundation Map DF#04.
- ^ Dibblee, Thomas. Geology of the central Santa Ynez Mountains, Santa Barbara County, California. Bulletin 186, California Division of Mines and Geology. San Francisco, 1966. 21
- ^ a b c d e f Dibblee (1966) 14
- ^ a b Dibblee, Thomas. Geology of Southwestern Santa Barbara County, California. Bulletin 150, California Division of Mines and Geology. San Francisco, 1950. 22
- ISBN 0-936494-35-2.
- ^ a b c Norris, 74
- ^ Thomas M. Dibblee, 1986. Geologic Map of the Little Pine Mountain Quadrangle, Santa Barbara County, California. California Department of Conservation, Division of Mines and Geology. Dibblee Foundation Map DF#05.
- ^ a b Dibblee (1966) 17
- ISBN 0-231-10585-1p. 218.