Stratigraphy of the Easternmost Ventura Basin, California, with a Description of a New Lower Miocene Mammalian Fauna from the Tick Canyon Formation

Abstract

The great thickness of dominantly nonmarine Tertiary strata in the eastern part of the Ventura Basin has impressed geologists since the time of the earliest surveys of the region. The younger of these strata, comprising the so-called Mint Canyon formation, have been eroded to form a great basinlike area in which numerous badland exposures occur (see fig. 1). Erosion has been due principally to the poor consolidation of the sediments and in part also to the rather scant cover of typical Sonoran vegetation. Although several earlier investigators examined these badlands, the first recorded discovery of vertebrate remains came as a result of reconnaissance mapping by Kew in 1919. In the publication that followed, a provisional list of the forms found here was furnished by Stock, although no detailed study of the fauna was made. After obtaining additional materials from the Mint Canyon beds, Maxson in 1930 published a detailed discussion of the fauna, and agreed with Stock on an upper Miocene age for the series. He further recognized that this vertebrate assemblage not only furnishes valuable data from which an age determination of the deposits might be made, but likewise sheds light on the age of the overlying marine strata and on their correlation with continental equivalents in the Great Basin and Great Plains provinces to the east. In a subsequent critical review of the fauna, Stirton reached conclusions not in accord with those of Maxson, and advocated a lower Pliocene age for the strata. Since that time, several papers of a mildly controversial nature have dealt directly or indirectly with the age of the Mint Canyon and with the stratigraphic relations of the fossils involved. The non-uniform character of the fauna, in which primitive and advanced types of mammals appear to be in association, has presented an additional complication. Both Maxson and Stirton have commented on this peculiar feature, although no stratigraphic break has been noted in the section.

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