EOCENE MARINE INVERTEBRATES FROM A LOCALITY
IN SIMI VALLEY, CALIFORNIA
by Jack D. Mount¹
One of the most complete sections of Eocene rocks in California may be found in the hills surrounding Simi Valley, Ventura County, California. Here over 3000 feet of clastic marine sediments crop out. These rocks contain a large and well preserved fossil fauna, which is as yet, except for a few papers describing new species of mollusks, largely unstudied. Unfortunately, most of the good collecting sites are on private property and permission to collect is not generally given. There is one locality, however, that is easily accessable and is the subject of this report.
The fossil locality is exposed in the Arroyo Simi which runs along the southern side of Santa Susana, a community at the eastern end of Simi Valley. The outcrop is 1750 feet west and 1160 feet north of the southeast corner of section 12, T 2 N, R 18 W, Santa Susana 7.5' Quadrangle (1951 edition). This site is registered in the Department of Geology, California State College, Los Angeles, as CSCLA Locality 605.
The middle Eocene in Simi Valley is represented by the siltstone, sandstone and conglomerate of the Llajas Formation. The fossil site is in interbedded sandstone and siltstone of this formation, 760 feet above the base.
In Table 1 may be found a list of fossils that have been recovered from Locality 605. The great abundance of Turritella andersoni lawsoni is characteristic of the layer. Of particular interest are the complete articulated specimens of the crab Plagiolophus weaveri which occur at the site. The following references are useful in identifying the fossils from the Llajas Formation: Vokes (1939), Clark and Vokes (1936), Turner (1938), Clark (1929), Hanna (1927), Stewart (1927, 1930), Merriam (1941), and Hanna and Hertlein (1941).
The molluscan fauna of this locality is closely related to a modern fauna that is restricted to a shallow tropical marine environment. The excellent condition of the fossils, especially the complete specimens of crustaceans, indicates that the ocean floor was free from strong turbulance.
The invertebrate fauna from this part of the Llajas Formation is typically middle Eocene in age (Domengine megafaunal stage). It has many species in common with the fauna of the Domengine Formation of central California (Clark, 1926, and Vokes, 1939) and with the fauna of the La Jolla Formation of San Diego County (Hanna, 1927).
Clark, B. L. 1926, The Domengine Horizon, middle Eocene of California: Univ. Calif. Pub. Geol. Sci., vol.16, no.5. p.99-118.
_____ 1929, Stratigraphy and Faunal Horizons of the Coast Ranges of California, privately published, Berkeley, 30 p., 50 pls.
_____ and Vokes, H. E., 1936, Summary of marine Eocene sequence of western North America: Geol. Soc. Amer. Bull., vol.47, no.6. p.851-878, 2 p1s.
Hanna, G. D., and Hertlein, L. G., 1941, Characteristic fossils of California; in Geologic formations and economic development of the oil and gas fields of California: Calif. Div. Mines Geol. Bull. 118, p.165-182, pls.60-67.
Hanna, M. A., 19270 An Eocene invertebrate fauna from the La Jolla Quadrangle, California; Univ. Calif. Pub. Geol. Sci., vol.16, no.8, p.247-398, 33 pls.
Merriam, C. W., 1941, Fossil turritellas from the Pacific coast region of North America: Univ. Calif. Pub. Geol. Sci., vol.26, no.l. p.1-214, 41 pls.
Stewart, R. B., 1927, Gabb's California fossil type gastropods: Proc. Acad. Nat. Sci. Philadelphia, vol.78, p.287-447, pls.20-32.
_____ 1930, Gabb's California Cretaceous and Tertiary type Lamellibranchs: Acad. Nat. Sci. Philadelphia Spec. Pub. 3, 314 p., 17 pls.
Turner, F. E., 1938, Stratigraphy and Mollusca of the Eocene of western Oregon: Geol. Soc. Amer. Spec. Paper 10, 130 p., 22 pls.
Vokes, H. E., 1939, Molluscan faunas of the Domengine and Arroyo Hondo Formations of the California Eocene: Annals New York Acad. Sci., vol.38, p.1-246, pls.1-22.
Table 1. Check list of middle Eocene marine invertebrate fossils fron the Llajas Formation in the Arroyo Simi, Santa Susana, Ventura County, California (CSCLA Locality 605).
Discocyclina cloptoni Vaughan
Architectonica cognata Gabb
Clavilithes tabulatus (Dickerson)
Conus hornii umpquaensis Turner
Cypraea (Eocypraea) castacensis Stewart
Eocernina hannibali (Dickerson)
Ficopsis reymondii crescentensis Weaver and Palmer
Fusiturricula crenatospira domenginica Vokes
Galeodea susanae Schenck
Rimella macilenta (White)
Tejonia clarki (Stewart)
Turritella andersoni lawsoni Dickerson
Turritella andersoni secondaria Merriam
Turritella applini M. A. Hanna
Volutocristata lajollaensis (M. A. Hanna)
Xenophora cf. X. stocki Dickerson
Corbula parilis Gabb
Nemocardium linteum (Conrad)
Ostrea idreaensis Gabb
Pecten (Polynemamusium) cf. P. interradiatus Gabb
Pecten (Polynemamusium) n.sp.
Plagiocardium (Schedocardia) brewerii (Gabb)
Pododesmus inornatus (Gabb)
Solena (Eosolen) coosensis Turner
Spondylus carlosensis Anderson
Trachycardium sorrentoense (M. A. Hanna)
Eogryphus tolmani Hertlein and Grant
Plagiolophus weaveri Rathbun
Spatangus cf. S. pachecoensis Pack
¹Department of Geology, University of California, Los Angeles, 90024.
Return to Paleontology Resources page
Return to first page of Jack's Home Place