Oreodonts from the Sespe deposits of South Mountain, Ventura County, California

Abstract

The Sespe deposits, a thick series of beds regarded as of non-marine origin, occupy a stratigraphic position in the Tertiary sequence of the Pacific Coast region between the Tejon marine Eocene and the Vaqueros marine Miocene. As mapped by W. S. W. Kew the Sespe beds are seen to possess an extensive distribution in southern California, occurring not only at the type locality on Sespe Creek, but are exposed also over broad areas along the borders of the Santa Clara Valley in Ventura County and in adjacent regions. Since their earliest study and description, these deposits have exhibited among other peculiarities an apparent absence of fossil remains. Because of the lack of palaeontological materials, determination of the age of the Sespe has depended upon its stratigraphic position and upon the recognition of the faunal stages of the marine invertebrate assemblages known from deposits which immediately underlie and overlie this accumulation. Since the Tejon is generally regarded as of Upper Eocene age and the Vaqueros as belonging to the Lower Miocene, various ages ranging from Eocene to Lower Miocene have been ascribed to the Sespe. The view has also been entertained that the period of accumulation of the Sespe accounts for much of geologic time that has elapsed since the Upper Eocene and prior to the Temblor stage of the Miocene. In recent years attention has been directed to the Sespe beds as exposed on the south side of the lower Santa Clara Valley near Santa Paula, California, as a result of the discovery of mammalian remains in these deposits by Dr. Nicolas L. Taliaferro. Following this discovery, further search in the area has brought to light additional material. The occurrence of mammalian remains is of considerable interest and importance since it affords an opportunity to present noteworthy data on the age of the Sespe and on the correlation of this accumulation with early Tertiary continental deposits of the Great Basin and Great Plains provinces. Furthermore, the materials available represent two distinct genera of oreodonts related to Promerycochrerus and Leptauchenia. This occurrence therefore records for the first time the presence of a type allied to the former genus west of the John Day region in north-central Oregon and of a member of the Leptaucheniar-Cyclopidius group west of the Great Plains.

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