Formations and Age--Subdivisions of West Coast Middle Tertiary: ABSTRACT

Equivalents of Lyell's Tertiary subdivisions were recognized early within the upper part of the West Coast marine sedimentary sequence. Boundaries between these series, however, have long remained controversial. The presence of the Eocene here was established clearly once the disputed presence of ammonites in the Pacific Coast Eocene had been resolved in the negative. The scene was thus set for the eventual recognition of Schimper's Paleocene in what had originally been termed lower Eocene Martinez, as distinguished from the first recognized, and higher, Eocene near Tejon Pass, California. The Pliocene-Pleistocene boundary soon was clarified, at least to the extent that this boundary had ever been clear in the typical terrain of Italy. On the West Coast, the tendency has een to place it between the two faunal zones in the Santa Barbara Formation (i.e., at the base of the original lower San Pedro), or more recently, following Woodring's recommendation, at the base of the Santa Barbara and its age-equivalents. In the middle Tertiary, however, the lower and upper boundaries of the Miocene, and subsequently of the Oligocene, were not as readily drawn. Some workers have placed the base of the Miocene as low as the lowest beds of the Vaqueros Formation, a Blakeley Oligocene age-equivalent and placed the top to include even the Pliocene Etchegoin Formation. The persistence on the West Coast of these middle Tertiary boundary problems has been due in part to the common presence in that stratigraphic interval of the organic Monterey Shale, with its sparse and commonly undiagnostic marine megafaunal assemblages. Another source of confusion, as pointed out by Schenck, has been the failure of many workers to note the qualification placed by Conrad on his age-evaluation of those megafossils found in the strata defined subsequently as the Vaqueros Formation; in effect, Conrad had simply said that those fossils indicated an age closer to Miocene than to the Eocene age of Blake's fossiliferous boulder from Tejon Pass. Finally, clarification of these middle Tertiary boundary problems was hampered further by the prevalent use of formational names fo time-rock subdivisions of Lyellian series-epochs. These subdivisions were defined on the basis of nothing more than the presence of a fauna or a species thought, on the basis of reconnaissance work, to be an "index species," at some zone within such a formation. Facies problems, too, were part of the trouble. The term "Temblor-Monterey" came into use when fossiliferous strata of these two formations were seen to interdigitate superpositionally. The major early work on the relations among the Monterey, Vaqueros, and Temblor Formations, attempted to resolve the problem by turning the Monterey into a middle Tertiary time-rock term of series magnitude. This legalism, both confusing and unnecessary, persisted for more than 20 years before the Monterey formally was restored to its original formational status by the U.S. Geological Survey. Meanwhile, terms such as Salinas and Maricopa had been coined for local developments of the Monterey Shale. Because microfossils are commonly abundant in the offshore, generally finer grained age-equivalents of the mollusk-bearing strata, micropaleontology has played a leading role in the clarification End_Page 439------------------------------ and redefinition of these series subdivisions. Thus, employing the criteria for biostratigraphic correlation summarized by Berry, the type San Lorenzo Formation has been shown to be Eocene at its lower zones, and equivalent in age to the Oligocene lower Vaqueros Formation in its upper beds. In turn, the Vaqueros at its type locality is seen to be in part age-equivalent to the generally still younger Temblor Formation. Locally, the lower parts of the Monterey Shale are correlative to parts of the Temblor, whereas stratigraphically higher intervals have been shown by the late George Richards to be age-equivalents of the type Santa Margarita Formation. Further, the type Santa Margarita is in part age-equivalent to the partly coarse clastic typical Modelo and Puente Formations of southern California, which in turn are partial age-equivalents of the organic shales of the Monterey. The type Santa Margarita is also in part age-equivalent to the Rodeo-Briones-San Pablo sequence of the Berkeley Hills. Refined microfaunal studies thus have made possible significant progress in our understanding of the complex stratigraphic relations of the West Coast middle Tertiary formations. A residue of confusion remains, however, which can be traced in part to the persistent use of reconnaissance "index species," as well as the dual use of names for the descriptive formational terminology and the interpretive time-rock terminology. End_of_Article - Last_Page 440------------